Motivation:
Facilitate the use of the NullAway checker as part of moving to
JSpecify.
Changes:
- represent "no children" as `List.of()` instead of null
- remove obsolete `children != null` assertions
- NullAway intentionally ignores such assertions
- remove "no children" special-casing where no longer necessary
Result:
- cleaner code with similar performance
- removed a barrier to using the NullAway checker