[Support] env vars with empty values on windows

An environment variable can be in one of three states:

1. undefined.
2. defined with a non-empty value.
3. defined but with an empty value.

The windows implementation did not support case 3
(it was not handling errors). The Linux implementation
is already correct.

Differential Revision: https://reviews.llvm.org/D36394

llvm-svn: 311174