[Tizen.System.Session] Ensure no-op (un)registration works correctly

[metiulekm]

Description of Change

The C# bindings coalesce multiple C# callbacks into one C callback, which is registered with the first C# callback and unregistered with the last C# callback. These operations should only happen on non-null<>null edges, otherwise there might be multiple registrations, or we can attempt unregistering when there is no registration.

However C# delegate removal and combination operators is actually more flexible than expected, in particular C# accepts nulls in both cases. Additionally, removal allows unregistering callbacks that are not registered, which is a no-op. These cases mean that some operations can start from a null and end in a null. In this case, we will call C callback (un)registration incorrectly.

This commit adds null checks:

• registrations check if the callback being registered is non-null,
  • if it is a non-null, the result will always be non-null,
  • if it is a null, it is safe to return because adding a null does not do anything,
• unregistrations check if the existing callback list is non-null,
  • it being non-null is enough to eliminate null<>null edges,
  • if it is a null, it is safe to return because there is nothing to unregister anyway.

API Changes

None

[JoonghyunCho]

🤖 [AI Review]

Reviewed — no findings.

Scope checked:

• Backport to API14 of the same change proposed for main in [Tizen.System.Session] Ensure no-op (un)registration works correctly #7849; fetched the full Session.cs at aaf6558 and confirmed the API14 file has the same 4 custom event accessors (AddUserWait, RemoveUserWait, SwitchUserWait, SwitchUserCompleted) at the same positions, so the 8 guard hunks apply identically and no event is left unguarded.
• add path: value == null early-return precedes the _handler == null registration check, so += null no longer triggers a native register on a null→null edge; a non-null value guarantees a non-null result, preserving register-on-first-subscriber.
• remove path: _handler == null early-return prevents -= x on an empty list from reaching UnregisterCallbackForEvent; removing a null or absent delegate from a non-null list leaves it non-null, so unregister still fires only on the true non-null→null edge.
• Both guards are inside the existing per-event lock, so edge detection remains atomic with the native call.
• RegisterCallbackForEvent throws via CheckError before _handler += value, so a failed native registration leaves the handler null and a later subscribe retries; unchanged and consistent with the new guard.
• Semantics now match the compiler-generated accessor behavior (+= null, -= null, removing an absent delegate are no-ops); no public API surface change.

No 🔴 critical issues, no 🟡 suggestions to flag.

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Automated review — final merge decision rests with human reviewers.