FIX: Generate Topic Thumbnails nil to Array error

If any value, including nil, is passed in as an argument the default
won't be set, so we need to handle when a non-Array value is passed in
to the `generate_thumbnails!` method.

app/models/topic.rb

@@ -95,6 +95,7 @@ class Topic < ActiveRecord::Base
     return nil unless SiteSetting.create_thumbnails
     return nil unless original = image_upload
     return nil unless original.width && original.height
+    extra_sizes = [] unless extra_sizes.kind_of?(Array)
 
     (Topic.thumbnail_sizes + extra_sizes).each do |dim|
       TopicThumbnail.find_or_create_for!(original, max_width: dim[0], max_height: dim[1])

spec/models/topic_thumbnail_spec.rb

@@ -7,7 +7,7 @@ describe "TopicThumbnail" do
 
   before do
     SiteSetting.create_thumbnails = true
-    topic.generate_thumbnails!
+    topic.generate_thumbnails!(extra_sizes: nil)
 
     TopicThumbnail.ensure_consistency!
     topic.reload