Make nsType optional in ChangeStreamDocument

[baileympearson]

Specifications typically use Optional<...> instead of | null to indicate that a field is only present in some circumstances because drivers represent conditionally present values differently. While some drivers might use null to indicate that a value isn't present, other drivers (like Node) omit keys entirely.

The changes in this PR make nsInfo consistent with other optional fields (ex: to or operationDescription).
Please complete the following before merging:

• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, sharded
  clusters, and serverless).