fix protocolbuffers#27: error when package name is nor present

lib/protobuf/protoc/generator/util.ex

@@ -8,6 +8,7 @@ defmodule Protobuf.Protoc.Generator.Util do
   end
 
   def attach_pkg(name, ""), do: name
+  def attach_pkg(name, nil), do: name
   def attach_pkg(name, pkg), do: normalize_pkg_name(pkg) <> "." <> name
 
   def options_to_str(opts) do

test/protobuf/protoc/generator/util_test.exs

@@ -0,0 +1,17 @@
+defmodule Protobuf.Protoc.Generator.UtilTest do
+  use ExUnit.Case, async: true
+
+  import Protobuf.Protoc.Generator.Util
+
+  test "attach_pkg can handle nil package" do
+    assert attach_pkg("name", nil) == "name"
+  end
+
+  test "attach_pkg can handle empty package" do
+    assert attach_pkg("name", "") == "name"
+  end
+
+  test "attach_pkg normolizes package name" do
+    assert attach_pkg("Name", "foo.bar") == "Foo.Bar.Name"
+  end
+end