False positive warning on switch statements with case null

[msridhar]

Example:

static Object handleNullableEnumCaseNullDefaultStatement(@Nullable NullableEnum nullableEnum) {
   Object result;
   switch (nullableEnum) {
     case null -> result = new Object();
     default -> result = nullableEnum.toString();
   };
   return result;
}

NullAway warns of a possible null dereference for the switch, but it should not, as there is a case null. (We currently don't warn on switch expressions as we don't match them yet due to google/error-prone#4119, but similar logic is required there.)