posts: fix bug preventing *_(cosplay) tags from being added.

Bug: if `hatsune_miku` is an empty general tag, then tagging a post with
`hatsune_miku_(cosplay)` would fail because `hatsune_miku` wasn't a
character tag. This could cause tagging posts to fail when aliasing a
character tag back to an old name that was used in the past, but was now
an empty general tag.

Fix: only treat the *_(cosplay) tag as being in conflict with the base
tag when the base tag is nonempty.

test/unit/post_test.rb

@@ -1296,6 +1296,14 @@ class PostTest < ActiveSupport::TestCase
           refute(@post.has_tag?("cosplay"))
           assert(@post.warnings[:base].grep(/Couldn't add tag/).present?)
         end
+
+        should "allow creating a _(cosplay) tag for an empty general tag" do
+          @tag = create(:tag, name: "hatsune_miku", post_count: 0, category: Tag.categories.general)
+          @post = create(:post, tag_string: "hatsune_miku_(cosplay)")
+
+          assert_equal("cosplay hatsune_miku hatsune_miku_(cosplay)", @post.reload.tag_string)
+          assert_equal(true, @tag.reload.character?)
+        end
       end
 
       context "that has been updated" do