Properly unset proxy env if empty #3271

Job	Run time
Resolve environment (ubuntu-latest, linked)	43s
Resolve environment (macos-latest, default)	2m 13s
Resolve environment (ubuntu-latest, default)	36s
Resolve environment (windows-latest, default)	3m 38s
Resolve environment (windows-latest, linked)	3m 31s
Resolve environment (windows-latest, nightly-latest)	2m 6s
Resolve environment (macos-latest, nightly-latest)	2m 30s
Resolve environment (ubuntu-latest, nightly-latest)	33s
Resolve environment (macos-latest, linked)	2m 13s
	18m 3s

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