go/types, types2: handle case of no specific target types in conversion

Avoid a panic by handling the case of no specific target type in a type
parameter to type parameter conversions.

Fixes #49864

Change-Id: I117dd80cc9d47c8c1e168f1caf0f281726270c84
Reviewed-on: https://go-review.googlesource.com/c/go/+/367616


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>