Return early in Classifier.classify if no languages supplied (#3471)

* Return early if no languages supplied

There's no need to tokenise the data when attempting to classify without a limited language scope as no action will be performed when it comes to scoring anyway.

* Add test for empty languages array

test/test_classifier.rb

@@ -53,4 +53,8 @@ class TestClassifier < Minitest::Test
       assert_equal language.name, results.first[0], "#{sample[:path]}\n#{results.inspect}"
     end
   end
+
+  def test_classify_empty_languages
+    assert_equal [], Classifier.classify(Samples.cache, fixture("Ruby/foo.rb"), [])
+  end
 end