cmd/go: update help for empty environment variables

Fixes #50335

Change-Id: I44b9dc6afa8c70b5cc8c79fb3ebddc3f45d3cef8
Reviewed-on: https://go-review.googlesource.com/c/go/+/475695
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: seankhliao

src/cmd/go/alldocs.go

@@ -471,9 +471,9 @@ var HelpEnvironment = &base.Command{
 	Long: `
 
 The go command and the tools it invokes consult environment variables
-for configuration. If an environment variable is unset, the go command
-uses a sensible default setting. To see the effective setting of the
-variable <NAME>, run 'go env <NAME>'. To change the default setting,
+for configuration. If an environment variable is unset or empty, the go
+command uses a sensible default setting. To see the effective setting of
+the variable <NAME>, run 'go env <NAME>'. To change the default setting,
 run 'go env -w <NAME>=<VALUE>'. Defaults changed using 'go env -w'
 are recorded in a Go environment configuration file stored in the
 per-user configuration directory, as reported by os.UserConfigDir.