mutable args changed None to empty tuple

[zsef123]

@gojomo
Issue(Closed) #1561
PR (Merged) #1562

In PR #1562, I changed empty list in default arguments to None and add check None
e.g)

def foo( arg=None):
    if arg is None:
        arg = []

But I requested under the comment

gojomo 5 hours ago Member
Making these defaults empty tuples – () – could enforce the immutability, and eliminate the later None-check/assignments.

So I change None to empty tuple () excepts graph.py.

In graph.py, can't apply these change.

[zsef123]

======================================================================
ERROR: testMostSimilarCosmul (gensim.test.test_fasttext_wrapper.TestFastText)
Test most_similar_cosmul for in-vocab and out-of-vocab words
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Traceback (most recent call last):
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/test/test_fasttext_wrapper.py", line 272, in testMostSimilarCosmul
    self.assertEqual(len(self.test_model.most_similar_cosmul(positive=['the', 'and'], topn=5)), 5)
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/models/word2vec.py", line 1250, in most_similar_cosmul
    return self.wv.most_similar_cosmul(positive, negative, topn)
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/models/keyedvectors.py", line 476, in most_similar_cosmul
    all_words = set([self.vocab[word].index for word in positive + negative
TypeError: can only concatenate list (not "tuple") to list
======================================================================
ERROR: test_cosmul (gensim.test.test_word2vec.TestWord2VecModel)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/test/test_word2vec.py", line 465, in test_cosmul
    sims = model.most_similar_cosmul('graph', topn=10)
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/models/word2vec.py", line 1250, in most_similar_cosmul
    return self.wv.most_similar_cosmul(positive, negative, topn)
  File "/home/travis/build/RaRe-Technologies/gensim/gensim/models/keyedvectors.py", line 476, in most_similar_cosmul
    all_words = set([self.vocab[word].index for word in positive + negative
TypeError: can only concatenate list (not "tuple") to list
----------------------------------------------------------------------

test failed