artists: fix accidental gentag category changes.

Bug: if you created an artist with the name of an existing general tag,
then the gentag would be changed to an artist tag, no matter how big the
gentag was.

Now we only allow creating artist entries for non-artist tags if the tag
is empty.

Ref: https://danbooru.donmai.us/forum_topics/17095

app/models/artist.rb

@@ -8,10 +8,12 @@ class Artist < ApplicationRecord
 
   before_validation :normalize_name
   before_validation :normalize_other_names
-  after_save :create_version
-  after_save :clear_url_string_changed
   validate :validate_tag_category
   validates :name, tag_name: true, uniqueness: true
+  before_save :update_tag_category
+  after_save :create_version
+  after_save :clear_url_string_changed
+
   has_many :members, :class_name => "Artist", :foreign_key => "group_name", :primary_key => "name"
   has_many :urls, :dependent => :destroy, :class_name => "ArtistUrl", :autosave => true
   has_many :versions, -> {order("artist_versions.id ASC")}, :class_name => "ArtistVersion"
@@ -151,14 +153,20 @@ class Artist < ApplicationRecord
 
   module TagMethods
     def validate_tag_category
-      return unless !is_deleted? && name_changed?
+      return unless !is_deleted? && name_changed? && tag.present?
 
-      if tag.category_name == "General"
-        tag.update(category: Tag.categories.artist)
-      elsif tag.category_name != "Artist"
+      if tag.category_name != "Artist" && !tag.empty?
         errors[:base] << "'#{name}' is a #{tag.category_name.downcase} tag; artist entries can only be created for artist tags"
       end
     end
+
+    def update_tag_category
+      return unless !is_deleted? && name_changed? && tag.present?
+
+      if tag.category_name != "Artist" && tag.empty?
+        tag.update!(category: Tag.categories.artist)
+      end
+    end
   end
 
   module BanMethods