fix: Add null checks when mapping nullable sources to non-nullable targets with MayBeNull and AllowNull Attribute

[Abhijithtv]

Add null checks when mapping nullable sources to non-nullable targets with MayBeNull Attribute

Description

For target members, [MaybeNull] should not be considered when determining nullability.
Otherwise, it may lead to incorrect nullable analysis and result in CS8601 ("Possible null reference assignment") warnings.

Fixes #2192

Checklist

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[latonz]

Thanks for the contribution! The fix turns out to be a bit more involved than it might initially appear, IMO also my original MayBeNull support PR was short-sighted. To handle this correctly, I think we need to support both MayBeNull (when reading values) and AllowNull (when writing values).

[Abhijithtv]

Thanks, that makes sense!

I checked and it looks like AllowNull isn't currently considered in target nullability checks (I also couldn’t find any tests covering it).

Would you prefer this to be handled in a separate PR focusing on AllowNull support?

Also, regarding the implementation: would you recommend modeling read and write nullability as separate properties (e.g., sourceMayBeNull vs targetAcceptsNull), or passing a context flag like isRead/isWrite?

[latonz]

As these two strongly belong together and the changes shouldn't get too big, I'd do it in the same PR.
I'd prefer separate properties/methods.

[latonz]

Thanks for the updates, I added my feedback.

Please also update the 5-0.md breaking changes docs.

[Abhijithtv]

@latonz Thanks for the review.

I realized I forgot to mark the PR as a draft earlier. Your feedback was really helpful.
I believe the PR is ready for review now.

[Abhijithtv]

@latonz
I have updated the PR based on the feedback.

[latonz]

Thanks for the updates. One small feedback point left.
Could you please also rebase? Afterwards I'll merge it 😊

[Abhijithtv]

Thanks for the updates. One small feedback point left. Could you please also rebase? Afterwards I'll merge it 😊

I have updated the PR and rebased the branch.

[latonz]

@Abhijithtv thank you for this contribution