Call to tzname() crashes when timezone name is None

Calling tzname() on a datetime object (in my case, the return value of dateutil.parser.parse()) crashes when the timezone name is None. The problem appears to be in tzname_in_python2() in tz.py, which returns the value of myfunc(*args, **kwargs).encode() to maintain consistency with Python 3 or something. The problem is when myfunc returns None, which throws "AttributeError: 'NoneType' object has no attribute 'encode'"

Storing the result of the call to myfunc and then checking if it's None before calling encode() appears to fix the problem. If it is None, then we can just return None, otherwise we can return result.encode() (more or less) safely.

I've personally changed my tz.py as follows because it's been preventing me from using a script I rely on:

- return myfunc(*args, **kwargs).encode()

+ result = myfunc(*args, **kwargs)
+ if not result:
+ return result
+ return result.encode()