extract and extractOne fail if choices contains null strings

[ciferkey]

Extract and extractOne fail if choices contains either an empty string or a string of only whitespace. For example:

process.extract('string1', ['', 'string', ''])

and

process.extract('string1', [' ', 'string', ' '])

will traceback:

    len_ratio = float(max(len(p1),len(p2)))/min(len(p1),len(p2))

ZeroDivisionError: float division by zero