Fixes piskvorky#1401 , Phrases behavious now consistent on different …

…versions , test added for empty iterator

gensim/models/phrases.py

@@ -115,7 +115,7 @@ def _is_single(obj):
         return True, obj_iter
     else:
         # If the first item isn't a string, assume obj is a corpus
-        return False, obj_iter
+        return False, list(obj_iter)
 
 
 class SentenceAnalyzer(object):

gensim/test/test_phrases.py

@@ -160,6 +160,13 @@ def setUp(self):
         self.bigram_unicode = Phrases(
             self.unicode_sentences, min_count=1, threshold=1, common_terms=self.common_terms)
 
+    def testEmptyPhrasifiedSentencesIterator(self):
+        bigram_phrases = Phrases(self.sentences)
+        bigram_phraser = Phraser(bigram_phrases)
+        trigram_phrases = Phrases(bigram_phraser[self.sentences])
+        trigram_phraser = Phraser(trigram_phrases)
+        self.assertNotEqual(trigram_phraser[bigram_phraser[self.sentences]].__len__(), 0)
+
     def testEmptyInputsOnBigramConstruction(self):
         """Test that empty inputs don't throw errors and return the expected result."""
         # Empty list -> empty list