If `PATH` is not defined, importing `comtypes` fails on Windows

[maxbelanger]

We've noticed that on some (rare) Windows devices, if PATH is not set or not readable when the application launches, comtypes crashes on import. The following stack trace illustrates the issue:

Traceback (most recent call last):
  <...>
  File "comtypes\client\__init__.pyc", line 21, in <module>
  File "comtypes\client\_events.pyc", line 8, in <module>
  File "comtypes\client\_generate.pyc", line 18, in <module>
KeyError: 'PATH'

Instead of expecting PATH to be defined, the code in _generate.py should fall back gracefully rather than break imports.