Fix an error when Python is not installed

[knyazer]

Description

I'm using a dev-container setup with distrobox, with everything besides the OS shared between the container and the host system. Python is not installed on the host system, but it is installed in the container. All vars are shared between the system and the container, so the host thinks that it is in the python environment, while the environment is not installed.

This fix shows nothing if there is no python available.

Motivation and Context

Shows error if VIRTUAL_ENV is well-defined, but python is not there.

Closes #

Screenshots (if appropriate)

How Has This Been Tested

Just checked on my machine; the change is trivial, so I don't think it needs heavy testing..

• I have tested using Linux.
• I have tested using MacOS.

Checklist

• I am ready to update the wiki accordingly. (not relevant)
• I have updated the tests accordingly.

[knyazer]

Oh, yikes, I just copied the code instead of formatting it well... will finish it in a few hours :)