Recommended change to join function

[EeroFluge]

Ref:

Python-2.7/Lib/posixpath.py

Line 61 in eaeb810

def join(a, *p):

I ran into a failure of this function when trying to upgrade OctoPrint. The error was 'NoneType' object has no attribute 'endswith'.

This was caused by the first input argument, a, being of type None. I added the following check:
path = a
if path == None:
path = ""

This fixed the problem and the OctoPrint upgrade finished correctly.