Adding venv check for VIRTUAL_ENV_PROMPT and assuring an empty 'prompt…

[philion]

…' field doens't erase a valid venv.

Specific issues:

1. No check for VIRTUAL_ENV_PROMPT.
2. Logic error looking for "prompt" after successfully loading "pyvenv.cfg", where that config file doesn't have that key defined. (Side note: Key() will create a new key with an empty value, according to the source.)

I have patched and tested "on my own machine".