Add empty separator check and tests

str.split method does not accept an empty string as a separator:

>>> 'foo'.split('')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: empty separator
>>>

Add an explicit test in the StringTrie constructor so we fail early when constructing the object.

(As an aside, str.split allows None as a separator. StringTrie could allow None separator but _key_from_path would have to be changed. If anyone needs that feature it’s fairly easy to implement).