cmd/go/internal/modload: fix (*mvsReqs).Max when the second argument is the empty string

As far as I can tell, this bug had gone unnoticed because everything
that uses Max so far happened to only ever present the empty string as
the first argument.

For #37438

Change-Id: Ie8c42313157d2c2c17e4058c53b5bb026b95a1c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/266860


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>