FIX: don't error Topic#similar_to when prepared raw is blank (PR #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.

GitHub

      expect(Topic.similar_to('some title', 'https://discourse.org/#INCORRECT#URI')).to be_empty