cmd/go: avoid panic on 'go mod' without arguments
cmd/go: avoid panic on 'go mod' without arguments

Fixes #26738

Change-Id: Icede3f59acb5b0e388660653cefc24a195b5d43b
GitHub-Last-Rev: 9989b74c12e94163c1f18859485a4ac0d74c5453
GitHub-Pull-Request: golang/go#26739
Reviewed-on: https://go-review.googlesource.com/127160
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

nawawi/go,nawawi/go,akutz/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,josharian/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,akutz/go,nawawi/go,golang/go,akutz/go,akutz/go,josharian/go,cloudflare/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,josharian/go,golang/go,golang/go,nawawi/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,josharian/go,josharian/go,nawawi/go