[dev.typeparams] cmd/compile/internal/types2: report error for invalid (but empty) expr switch

Enable one more errorcheck test.

Updates #43110.
Updates #43200.

Change-Id: Ib7b971d5e9989c65320579f75d65266bbbbeec53
Reviewed-on: https://go-review.googlesource.com/c/go/+/278132


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>