`GPBDictionary` is incorrectly marking `objectForKey:` as `nonnull ` due to `NS_ASSUME_NONNULL_BEGIN` Macro

[luoshuaiqing]

What version of protobuf and what language are you using?
Version: 28.3
Language: Objective-C

What operating system (Linux, Windows, ...) and version?
MacOS

What did you do?
Root Cause:

• GPBDictionary is incorrectly marking objectForKey: as nonnull due to NS_ASSUME_NONNULL_BEGIN Macro

Problem it caused for me:

• When I call this method from swift, it will crash the App if the key doesn't exist in the GPBDictionary because objc marks it as nonnull while it is actual returning null, and therefore Swift recognizes as a non-optional object while it is actually a nil.