Support RUNFILES_MANIFEST_FILE Set to an Empty Value

[levymp]

Currently, when using the runfiles library in an environment where RUNFILES_MANIFEST_FILE is provided, but set to an empty value results in an error. The rules_python runfiles library supports the environment variable being set to an empty value.

The rules_rust runfiles library should also support an empty RUNFILES_MANIFEST_FILE value and fallback on a directory based approach.

[UebelAndre]

@fmeum do you know if this is the correct behavior for runfiles?

[fmeum]

@fmeum do you know if this is the correct behavior for runfiles?

I would say it doesn't really matter. Bazel itself never sets the variables to an empty value. Treating empty variables as equivalent to them not being set is pretty common, I guess.