CSHARP-6066: Fix NullReferenceException in FilteredMongoCollectionBase.FindOneAndUpdate when options is null

[flibustier7seas]

Summary

Fixes https://jira.mongodb.org/browse/CSHARP-6066

All four FindOneAndUpdate / FindOneAndUpdateAsync overloads in FilteredMongoCollectionBase access options.IsUpsert without a null check, despite the options parameter defaulting to null. This causes a NullReferenceException when callers rely on the default value, e.g. collection.OfType<Derived>().FindOneAndUpdate(filter, update).

Fix

Replace options.IsUpsert with options?.IsUpsert ?? false in all four overloads. This is consistent with how IsUpsert defaults to false in FindOneAndUpdateOptions, and matches the null-tolerant pattern used elsewhere in the same file (e.g. MapReduce does options = options ?? new ...).

Testing

Added FindOneAndUpdate_should_not_throw_when_options_is_null unit test covering both sync and async paths.

[Copilot]

Pull request overview

Fixes a NullReferenceException in FilteredMongoCollectionBase.FindOneAndUpdate* overloads when callers pass null (or rely on the default null) for FindOneAndUpdateOptions, by making IsUpsert access null-safe. This aligns the filtered collection wrapper behavior with the underlying driver implementations where IsUpsert defaults to false.

Changes:

• Updated all 4 FindOneAndUpdate / FindOneAndUpdateAsync overloads to use options?.IsUpsert ?? false when adjusting the update definition.
• Added a unit test ensuring OfType-filtered collections do not throw when options is null (sync and async entrypoints).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/MongoDB.Driver/FilteredMongoCollectionBase.cs	Prevents NRE by making IsUpsert access null-safe in FindOneAndUpdate* wrapper overloads.
tests/MongoDB.Driver.Tests/OfTypeMongoCollectionTests.cs	Adds regression coverage to ensure FindOneAndUpdate* does not throw when options are null.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[BorisDog]

We shouldn't check for null exception.
@papafe do you think we can add this as a rule for review agents?

[flibustier7seas]

We shouldn't check for null exception.

Fixed