BURs: don't check for category when implicating empty tags

The default category for empty tags is general, so implications are otherwise
always going to be rejected when the tag is created via BUR beforehand.

Ref: https://danbooru.donmai.us/forum_topics/18013?page=2#forum_post_182107

app/models/tag_implication.rb

@@ -99,7 +99,10 @@ class TagImplication < TagRelationship
       end
     end
 
+    # Require tags to have the same category. Doesn't apply when either tag is empty,
+    # because they could have been populated from a previous update script.
     def tag_categories_are_compatible
+      return if antecedent_tag.empty? || consequent_tag.empty?
       if antecedent_tag.category != consequent_tag.category
         errors.add(:base, "Can't imply a #{antecedent_tag.category_name.downcase} tag to a #{consequent_tag.category_name.downcase} tag")
       end