Add nullability annotations for all private headers

[nlutsenko]

We currently have 100% nullability annotations for all public headers, but there are quite a lot of private ones that are missing this.
This issue is to track addition of nullability annotations to all of them.
The plan for every one of those headers is as follows:

• Use NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END for entire header
• Add nullable to all types that can be nil to everything else
• ???
• Profit

[stale]

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[parse-github-assistant]

The label type:feature cannot be used in combination with type:improvement.