Respond with valid json when relationship is nil

```
    book = Book.new(author: nil)
    book.as_json(includes: [:author])

    {
        relationships: {
            author: { data: { meta: {} } }
        }
    }
```

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -21,7 +21,7 @@ def initialize(parent_serializer, serializer, serializable_resource_options, arg
 
         def as_json
           hash = {}
-          hash[:data] = data || {} if association_options[:include_data]
+          hash[:data] = data || { meta: {} } if association_options[:include_data]
           links = self.links
           hash[:links] = links if links.any?
           meta = self.meta

test/action_controller/json_api/linked_test.rb

@@ -119,7 +119,7 @@ def test_render_resource_with_nested_has_many_include
               'relationships' => {
                 'posts' => { 'data' => [] },
                 'roles' => { 'data' => [{ 'type' => 'roles', 'id' => '1' }, { 'type' => 'roles', 'id' => '2' }] },
-                'bio' => { 'data' => nil }
+                'bio' => { 'data' => { 'meta' => {} } }
               }
             }, {
               'id' => '1',

test/adapter/json_api/belongs_to_test.rb

@@ -75,7 +75,7 @@ def test_include_nil_author
           serializer = PostSerializer.new(@anonymous_post)
           adapter = ActiveModelSerializers::Adapter::JsonApi.new(serializer)
 
-          assert_equal({ comments: { data: [] }, blog: { data: { type: 'blogs', id: '999' } }, author: { data: nil } }, adapter.serializable_hash[:data][:relationships])
+          assert_equal({ comments: { data: [] }, blog: { data: { type: 'blogs', id: '999' } }, author: { data: { meta: {} } } }, adapter.serializable_hash[:data][:relationships])
         end
 
         def test_include_type_for_association_when_different_than_name
@@ -119,7 +119,7 @@ def test_include_linked_resources_with_type_name
               relationships: {
                 posts: { data: [] },
                 roles: { data: [] },
-                bio: { data: nil }
+                bio: { data: { meta: {} } }
               }
             }, {
               id: '42',
@@ -143,7 +143,7 @@ def test_include_linked_resources_with_type_name
               relationships: {
                 comments: { data: [] },
                 blog: { data: { type: 'blogs', id: '999' } },
-                author: { data: nil }
+                author: { data: { meta: {} } }
               }
             }
           ]

test/adapter/json_api/has_many_explicit_serializer_test.rb

@@ -78,7 +78,7 @@ def test_includes_author_id
         def test_explicit_serializer_with_null_resource
           @post.author = nil
 
-          expected = { data: nil }
+          expected = { data: { meta: {} } }
 
           assert_equal(expected, @adapter.serializable_hash[:data][:relationships][:author])
         end

test/adapter/json_api/has_many_test.rb

@@ -50,7 +50,7 @@ def test_includes_linked_comments
             },
             relationships: {
               post: { data: { type: 'posts', id: '1' } },
-              author: { data: nil }
+              author: { data: { meta: {} } }
             }
           }, {
             id: '2',
@@ -60,7 +60,7 @@ def test_includes_linked_comments
             },
             relationships: {
               post: { data: { type: 'posts', id: '1' } },
-              author: { data: nil }
+              author: { data: { meta: {} } }
             }
           }]
           assert_equal expected, @adapter.serializable_hash[:included]
@@ -73,14 +73,14 @@ def test_limit_fields_of_linked_comments
             type: 'comments',
             relationships: {
               post: { data: { type: 'posts', id: '1' } },
-              author: { data: nil }
+              author: { data: { meta: {} } }
             }
           }, {
             id: '2',
             type: 'comments',
             relationships: {
               post: { data: { type: 'posts', id: '1' } },
-              author: { data: nil }
+              author: { data: { meta: {} } }
             }
           }]
           assert_equal expected, @adapter.serializable_hash[:included]

test/adapter/json_api/linked_test.rb

@@ -92,7 +92,7 @@ def test_include_multiple_posts_and_linked_array
                 },
                 relationships: {
                   post: { data: { type: 'posts', id: '10' } },
-                  author: { data: nil }
+                  author: { data: { meta: {} } }
                 }
               }, {
                 id: '2',
@@ -102,7 +102,7 @@ def test_include_multiple_posts_and_linked_array
                 },
                 relationships: {
                   post: { data: { type: 'posts', id: '10' } },
-                  author: { data: nil }
+                  author: { data: { meta: {} } }
                 }
               }, {
                 id: '1',
@@ -274,7 +274,7 @@ def test_nil_link_with_specified_serializer
               },
               relationships: {
                 comments: { data: [{ type: 'comments', id: '1' }, { type: 'comments', id: '2' }] },
-                author: { data: nil }
+                author: { data: { meta: {} } }
               }
             }
           }

test/adapter/json_api/relationship_test.rb

@@ -23,13 +23,13 @@ def test_relationship_with_data
 
         def test_relationship_with_nil_model
           @serializer = BlogSerializer.new(nil)
-          expected = { data: nil }
+          expected = { data: { meta: {} } }
           test_relationship(expected, options: { include_data: true })
         end
 
         def test_relationship_with_nil_serializer
           @serializer = nil
-          expected = { data: nil }
+          expected = { data: { meta: {} } }
           test_relationship(expected, options: { include_data: true })
         end