fix: Type hinting MPIdentityApiRequest.identities

[einsteinx2]

Summary

Fix crash when accessing the -[MPIdentityApiRequest identity] property from Swift due to incorrect type hinting

We had it hinted as <NSNumber*, NSString*> but then we also stored NSNull objects in some cases. That was fine with Objective-C since it ignores the type hints at runtime, but Swift doesn't and crashes if any NSNull objects are present when accessing the identities property.

Note this doesn't actually change the underlying behavior of this property in any way, it just now is correctly type hinted as <NSNumber*, NSObject*> to include the possibility of NSNull.

Testing Plan

• [x] Was this tested locally? If not, explain why.

• First reproduced the crash, then confirmed that I can now access the property from Swift without crashing. Also added unit tests to confirm both the core behavior and the Swift interop, explicitly accessing the property from Swift when it contains an NSNull object.

  Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/SQDSDKS-6338

[mparticle-automation]

:tada: This PR is included in version 8.21.1 :tada:

The release is available on GitHub release

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