[vm/aot] Include entries for null in the dispatch table to avoid check.

NullCheck pc descriptors are added to all dispatch table calls where the
receiver may be null (and the selector is not one implemented by null).

All null entries in the table go to the NullError runtime entry, which
reads the NullCheck pc descriptor to get the name of the called member
for the error message.

Change-Id: I9d2847d0ccdfdb735b06e879916920ec299f39bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134294
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>