Fix error when unsetting nonexistent environment variable in tests
Fix error when unsetting nonexistent environment variable in tests

dimfeld/gsutil,dtjackson/gsutil,GoogleCloudPlatform/gsutil,mattdr/gsutil,fishjord/gsutil,craigcitro/gsutil,chriskuehl/gsutil-debian,GoogleCloudPlatform/gsutil,BrandonY/gsutil,ttiurani/gsutil