cmd/go: fix go work sync when there are zero workspace modules

go work sync panics when there are no workspace modules. This is
because the code that set the pruning mode only did so with modules
present. This change changes pruningForGoVersion to properly return
workspace pruning in workspace mode to prevent that. Another weird
scenario can happen when there are no workspace modules, but the
command-line-arguments module is created by default. Check for that
when iterating over the workspace modules to avoid trying to find the
nonexistant go.mod file for that modules.

Fixes #49591

Change-Id: Iee8bc92a8aaf9c440f88fe4f9ca908a8d461cd36
Reviewed-on: https://go-review.googlesource.com/c/go/+/365234


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