Improve error recovery for null coalescing assignment operator

[DoctorKrolic]

Part of compiler part of #73894 (the issue also mentiones operators like +=, -= etc., but I focused only on ??= in this PR to keep it relatively small)

While working on PR I found several inconsistencies in what compiler APIs return in normal and error scenarios. These are marked with // INCONSISTENCY: comments in tests. We need to decide what the desired behavior should be and either:

• Remove/alter the comments if in the given case the behavior is decided to be expected
• Make an issue (or several ones) for cases, that can/should be improved ...
• ... or alternatively fix them in this PR (which I don't like given that the total size of this PR is already around ~1000 lines with actual changes taking just ~20 lines of the codebase)