Check arrays are empty instead of size 0

In ruby we check for `array.empty?` instead of `array.size == 0`, so
it's natural to write the tests in the same way.

spec/models/debate_spec.rb

@@ -762,7 +762,7 @@ describe Debate do
     it "does not return any debates when user has not interests" do
       create(:debate)
 
-      expect(Debate.recommendations(user).size).to eq 0
+      expect(Debate.recommendations(user)).to be_empty
     end
 
     it "returns debates ordered by cached_votes_total" do