model.trainables.update_weights executes even when no new vocabulary is added

This one is not a major issue, but I think it is important to fix in order to avoid confusion.

Even when there is no new vocabulary, model.trainables.update_weights takes a while to execute (with no effect on the model) because it doesn't check whether gained_vocab is greater than zero or not.

https://github.com/RaRe-Technologies/gensim/blob/06f5f5c4fa9fb54a169e53034a3bf3fa035cbc3c/gensim/models/word2vec.py#L1467

should be replaced by:

gained_vocab = len(wv.vocab) - len(wv.vectors)
if gained_vocab <= 0:
	raise RuntimeError(
	    "There is no new vocabulary added.  "
	    "If you want to update the weights with a new vocabulary, please first update model.vocab"
	)