Don't print query params if they are empty

I found this while investigating this issue on Lucky:
luckyframework/lucky#214

It turns out that because I parsed the query params in that handler it
set the URI#query to `""`. This caused `HTTP::Request#resource` to add
an extra `?` to the end of the path, even though the query params were
empty.

This PR fixes the issue by also checking for an empty string before
printing `?`

spec/std/uri_spec.cr

@@ -32,12 +32,20 @@ describe "URI" do
   assert_uri("/foo?q=1", path: "/foo", query: "q=1")
   assert_uri("mailto:[email protected]", scheme: "mailto", path: nil, opaque: "[email protected]")
 
-  it { URI.parse("http://www.example.com/foo").full_path.should eq("/foo") }
-  it { URI.parse("http://www.example.com").full_path.should eq("/") }
-  it { URI.parse("http://www.example.com/foo?q=1").full_path.should eq("/foo?q=1") }
-  it { URI.parse("http://www.example.com/?q=1").full_path.should eq("/?q=1") }
-  it { URI.parse("http://www.example.com?q=1").full_path.should eq("/?q=1") }
-  it { URI.parse("http://test.dev/a%3Ab").full_path.should eq("/a%3Ab") }
+  describe "full_path" do
+    it { URI.parse("http://www.example.com/foo").full_path.should eq("/foo") }
+    it { URI.parse("http://www.example.com").full_path.should eq("/") }
+    it { URI.parse("http://www.example.com/foo?q=1").full_path.should eq("/foo?q=1") }
+    it { URI.parse("http://www.example.com/?q=1").full_path.should eq("/?q=1") }
+    it { URI.parse("http://www.example.com?q=1").full_path.should eq("/?q=1") }
+    it { URI.parse("http://test.dev/a%3Ab").full_path.should eq("/a%3Ab") }
+
+    it "does not add '?' to the end if the query params are empty" do
+      uri = URI.parse("http://www.example.com/foo")
+      uri.query = ""
+      uri.full_path.should eq("/foo")
+    end
+  end
 
   describe "normalize" do
     it "removes dot notation from path" do

src/uri.cr

@@ -128,7 +128,7 @@ class URI
   def full_path
     String.build do |str|
       str << (@path.try { |p| !p.empty? } ? @path : "/")
-      str << "?" << @query if @query
+      str << "?" << @query if (query = @query) && !query.empty?
     end
   end