[ddc] Handle `undefined` as `null` in switch

Add a case for `undefined` whenever a case expression is a null
constant.

Issue: https://github.com/dart-lang/sdk/issues/51527
Change-Id: I6e9946b588e293bcf0b953d568495c8be95f3749
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310775
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

https://dart.googlesource.com/sdk/+/cc5b77a71045302005743d5ae871974dcc679423