🐛 🔧 fix Zappa.create_handler_venv win32 case wherestderror_result is None

[monkut]

Description

Update Zappa.create_handler_venv, so that when the result from pip_process.communicate() returns None for stderr, an unrelated exception is NOT raised, and zappa.create_handler_venv proceeds as expected.

Currently, the code assumes a str value is returned, but on win32 systems, it appears that None may be returned.

GitHub Issues

#1342

[coveralls]

coverage: 74.783% (+0.1%) from 74.648%
when pulling c9a7b79 on fix/win-slimhandler-error-handling-issue1342
into b00312d on master.