cmd/go: fix missing case checking for empty slice

cmd/go: fix missing case checking for empty slice

When we were comparing the first element of import stacks when sorting
depserrors we checked if the first stack was non empty, but not the
second one. Do the check for both stacks.

Fixes #61816
For #59905

Change-Id: Id5c11c2b1104eec93196a08c53372ee2ba97c701
Reviewed-on: https://go-review.googlesource.com/c/go/+/516739
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>