[release-branch.go1.17] go/types: fix panic with nil package name

This is a straightforward port of CL 320490 to go/types.

Fixes #48082

Change-Id: I763c806c777f926a563d8f9384764e5b3f7f083c
Reviewed-on: https://go-review.googlesource.com/c/go/+/324732


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
(cherry picked from commit e32fab14)
Reviewed-on: https://go-review.googlesource.com/c/go/+/347071