Fix mapping nil transformer output back to NSNull

[dtrofimov]

Bug: if a transformer encodes a value to nil, it's converted to a missing value, not to a null value.

Example. My model has an integer value. In case of equality to INVALID_VALUE constant, it should be encoded as null. So I implement the transformer as follows:

return [MTLValueTransformer transformerUsingForwardBlock:^id(NSNumber *num, BOOL *success, NSError *__autoreleasing *error) {
    return num?: @(INVALID_VALUE);
} reverseBlock:^id(NSNumber *num, BOOL *success, NSError *__autoreleasing *error) {
    return (num.intValue == INVALID_VALUE)? nil: num;
}];

I convert a structure with an invalid value in a following manner: model → JSON → model. And the value is transformed as INVALID_VALUE → absent value → 0 (not assigned).

The reason is a mistake in MTLJSONAdapter. As stated in comments, it maps NSNull → nil to use the transformer, and then back after the transformer. But the reverse mapping isn't performed in this specific case.

[github-actions]

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