reltags: fix nil exception on fast_count timeouts.

Fixes a nil dereference error when calculating related tags for the
sidebar. Cased by Post.fast_count returning nil when it times out.

app/logical/related_tag_calculator.rb

@@ -1,6 +1,8 @@
 module RelatedTagCalculator
   def self.similar_tags_for_search(tag_query, search_sample_size: 1000, tag_sample_size: 250, category: nil)
     search_count = Post.fast_count(tag_query)
+    return [] if search_count.nil?
+
     search_sample_size = [search_count, search_sample_size].min
     return [] if search_sample_size <= 0