feat: Document/test the behavior of passing an empty string to SetEnv on Windows

[ghost]

Fixes #3027

This is a quick PR to implement my suggestion. Note that I'm not 100% behind this idea because it may be surprising that the env var isn't actually empty when using SetEnv(var, "") on Windows.

This documents the Windows behavior and adds some tests.

cc @tmatsuo

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This change is 

[tmatsuo]

@remyabel Thanks, LGTM, but I'd like to hear from @coryan too.

[tmatsuo]

Maybe you can tweak the title, because it just adds some comments?

[codecov]

Codecov Report

Merging #3030 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3030      +/-   ##
==========================================
+ Coverage   90.88%   90.96%   +0.07%     
==========================================
  Files         300      300              
  Lines       20326    20290      -36     
==========================================
- Hits        18474    18457      -17     
+ Misses       1852     1833      -19

Impacted Files	Coverage Δ
google/cloud/storage/oauth2/credentials.h	33.33% <0%> (-33.34%)	⬇️
google/cloud/storage/internal/complex_option.h	66.66% <0%> (-16.67%)	⬇️
...oogle/cloud/bigtable/internal/unary_client_utils.h	84.37% <0%> (-15.63%)	⬇️
google/cloud/storage/internal/curl_handle.h	65.62% <0%> (-12.5%)	⬇️
google/cloud/bigtable/internal/common_client.h	89.47% <0%> (-10.53%)	⬇️
google/cloud/storage/well_known_parameters.h	70% <0%> (-8.89%)	⬇️
google/cloud/storage/well_known_headers.h	77.77% <0%> (-7.41%)	⬇️
...ud/storage/examples/storage_client_mock_samples.cc	86.24% <0%> (-4.24%)	⬇️
google/cloud/storage/internal/generic_request.h	86.48% <0%> (-2.71%)	⬇️
...gle/cloud/storage/internal/curl_request_builder.cc	96.2% <0%> (ø)	⬆️
... and 29 more

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[coryan]

@remyabel @tmatsuo I definitely do not understand something here, because the tests are failing.

[ghost]

The tests were failing because I forgot to modify the other tests. I think in the second log this error came up again.

[coryan]

Reviewed 2 of 4 files at r1, 1 of 2 files at r2.
Reviewable status: 3 of 4 files reviewed, 2 unresolved discussions (waiting on @remyabel)

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google/cloud/internal/env_test.cc, line 35 at r3 (raw file):

/// @test Verify we can unset an environment variable with nullptr.
TEST(SetEnv, UnsetEnvWithNullptr) {
  SetEnv("foo", "");

We could simplify this test by using SetEnv("foo", "bar"), right? We already checked that setting to an empty string does the expected thing in the previous test?

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google/cloud/internal/env_test.cc, line 47 at r3 (raw file):

/// @test Verify we can unset an environment variable.
TEST(SetEnv, UnsetEnv) {
  SetEnv("foo", "");

Ditto?

[coryan]

Thanks!

Reviewed 1 of 1 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved