go/types, types2: report empty type sets in operand descriptions

go/types, types2: report empty type sets in operand descriptions

This leads to better error messages where operations are not
permitted because of empty type sets.

Fixes #51525.

Change-Id: I8d15645e2aff5145e458bdf9aaa4d2bee28d37fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/452535
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>