[vm] Avoid sentinel entry at the end of invocation_dispatcher_cache.
[vm] Avoid sentinel entry at the end of invocation_dispatcher_cache.

No code actually relies on it being there.

This saves ~1kb of snapshot size on a large Flutter application.

Related to https://github.com/dart-lang/sdk/issues/48910

TEST=ci

Change-Id: I03cf8811f39fc8882f7c650a79f01d350b520e5d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/242863
Commit-Queue: Slava Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk