Fixed ZeroDivisionError when UA contains punctuation only

[anatoliifetisov]

There's an edge case when alphabetic_chars / len(app_no_punc) < threshold fails with ZeroDivisionError. It's extremely rare in real-world data (as in "couple of dozens per hundreds of millions"-rare), but quite destructive nevertheless.
One of the inputs that cause this is "%7C%7C%27", which decodes into "||'" and leads (after punctuation stripping) to 0/0 evaluation.

Not sure if an empty string should result in True or False to be returned, but since True effectively forces the whole thing to be discarded (and such strings should definitely be discarded) it makes more sense to me.

[anatoliifetisov]

Another example of a useragent that breaks stuff is "Mozilla/5.0 (Linux; Android 5.1.1; LGLS751 Build/????? ?/ ????????? ???????_??:-775296273) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/43.0.2357.93 Mobile Safari/537.36", ZeroDivisionError happens when the parser is trying to figure out secondary client data.