Permit empty environment variables as unset in setup.py

[loadams]

This change fixes a bug that causes setup.py to fail when any of its configuration environment variables are the empty string.

For example, running the following:

export DS_BUILD_OPS=""
pip install deepspeed

Currently fails with the following message:

BUILD_OP_DEFAULT = int(os.environ.get('DS_BUILD_OPS', BUILD_OP_PLATFORM))
ValueError: invalid literal for int() with base 10: ''

Because os.environ.get does not use its fallback argument when an environment variable exists, but is empty.
Similarly, key in os.environ returns True for empty (but still set) keys.

Shells usually do not distinguish between unset variables and empty variables, so setting variables to the empty string is usually seen as another way to unset them. This specifically arose as an issue for me while I was writing a Dockerfile, since those have no way to unset ENV and ARG directives—only to set them to the empty string (or to figure out and hardcode what the default would have resolved to for each ARG). I figure fixing this here is more helpful than writing weird workarounds.

This changes setup.py to treat unset and empty environment variables identically. This behaviour could potentially be changed in other files, as well, but that is not included here.

Co-authored-by: Eta0 Eta0@users.noreply.github.com

[loadams]

This is a follow up from #3763