Purge invalid gentags from tags table (#3390).

Purges gentags with invalid names from the tags table. Addresses #3390
(Searching certain metatags results in an empty paginator) by removing
metatags from the tags table.

app/models/tag.rb

@@ -783,6 +783,10 @@ class Tag < ApplicationRecord
   end
 
   module SearchMethods
+    def empty
+      where("tags.post_count <= 0")
+    end
+
     def nonempty
       where("tags.post_count > 0")
     end

script/fixes/051_purge_invalid_tags.rb

@@ -0,0 +1,20 @@
+#!/usr/bin/env ruby
+
+require File.expand_path(File.join(File.dirname(__FILE__), '..', '..', 'config', 'environment'))
+
+CurrentUser.user = User.system
+CurrentUser.ip_addr = "127.0.0.1"
+
+Tag.transaction do
+  empty_gentags = Tag.empty.where(category: Tag.categories.general)
+  total = empty_gentags.count
+
+  empty_gentags.find_each.with_index do |tag, i|
+    STDERR.puts %{validating "#{tag.name}" (#{i}/#{total})} if i % 1000 == 0
+
+    if tag.invalid?(:create)
+      # puts ({ name: tag.name, id: tag.id }).to_json
+      tag.delete
+    end
+  end
+end