Guard against classify nil data

lib/linguist/classifier.rb

@@ -97,6 +97,7 @@ module Linguist
     # Returns sorted Array of result pairs. Each pair contains the
     # Language and a Float score.
     def classify(tokens, languages = @languages.keys)
+      return [] if tokens.nil?
       tokens = Tokenizer.new(tokens).tokens if tokens.is_a?(String)
 
       scores = {}

test/test_classifier.rb

@@ -55,6 +55,10 @@ class TestClassifier < Test::Unit::TestCase
     assert results.first[1] < 0.5, results.first.inspect
   end
 
+  def test_instance_classify_nil
+    assert_equal [], Classifier.instance.classify(nil)
+  end
+
   def test_verify
     assert Classifier.instance.verify
   end