[PLAT-6896] Fix NSNull handling

[nickdowell]

Goal

Fix a crash that can occur when uploading events that contain a null / NSNull stacktrace value.

NSInvalidArgumentException: -[NSNull countByEnumeratingWithState:objects:count:]: unrecognized selector sent to instance 0x20a654a00

0  CoreFoundation          ___exceptionPreprocess
1  libobjc.A.dylib         _objc_exception_throw
2  CoreFoundation          -[NSObject(NSObject) doesNotRecognizeSelector:]
3  CoreFoundation          ____forwarding___
4  CoreFoundation          ___forwarding_prep_0___
5  Runner                  +[BugsnagError errorFromJson:] (BugsnagError.m:119:9)
6  Runner                  -[BugsnagEvent stacktraceTypes] (BugsnagEvent.m:729:31)
7  Runner                  -[BSGEventUploadOperation runWithDelegate:completionHandler:] (BSGEventUploadOperation.m:112:67)
8  Runner                  -[BSGEventUploadOperation start] (BSGEventUploadOperation.m:177:5)

See #1138

Changeset

+[BugsnagError errorFromJson:] and +[BugsnagStackframe frameFromJson:] now use the BSGDeserialize... functions to check the value types when deserializing from JSON.

BSGEventUploadOperation now encapsulates the main logic in a @try ... @catch block to prevent crashes, and reports any caught exceptions to Bugsnag using the internal error reporting mechanism.

Testing

Unit tests have been amended to verify correct handling of NSNull values.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 1,152 bytes from 1,132,872 to 1,134,024

Generated by 🚫 Danger