FIX: don't error Topic#similar_to when prepared raw is blank (#10711)

If raw contains incorrect URL, `prepare_data` returns empty string:

https://github.com/discourse/discourse/blob/master/lib/search.rb#L91

Therefore we should not only check if the cooked post is not blank but also if prepared data is not blank.

app/models/topic.rb

@@ -597,9 +597,11 @@ class Topic < ActiveRecord::Base
         PrettyText.cook(raw[0...MAX_SIMILAR_BODY_LENGTH].strip)
       )
 
-      if cooked.present?
+      prepared_data = cooked.present? && Search.prepare_data(cooked)
+
+      if prepared_data.present?
         raw_tsquery = Search.set_tsquery_weight_filter(
-          Search.prepare_data(cooked),
+          prepared_data,
           'B'
         )