Remove stripping of null values from Selection Set models

[AnthonyMDev]

This PR removes the stripping of null values from the selection set models.

This fixes apollographql/apollo-ios#3092. Because data returned from a network request was having null values stripped, when the returned models were then manually written to the cache, the null values were not written. This results in a missingValue error when you try to read those objects from the cache later.

This situation could occur if you fetched from the network with a cache policy of .fetchIgnoringCacheCompletely and then wrote some of the data to the cache manually. It also occurs in the common case of a fetched object be appended to another list of objects in a cache mutation.

Null Value Representation

This bug fix revealed another related bug. We previously used NSNull to represent null values in a SelectionSet, then in order to work for Swift 5.3 and below, we needed to change this. We changed it to Optional<AnyHashable>.none, which actually was just nil. These nil values were being stripped by the executor as well.

In order to handle null values in the selection set appropriately, we need to store them as a concrete value. This is now represented as AnyHashable(Optional<AnyHashable>.none), which wraps a nil value inside of an AnyHashable struct. This is a concrete value instead of just nil, which provides the desired behavior.

To make the code clearer and more expressive, I've created a DataDict.NullValue static property that contains this value.

[BobaFetters]

This LGTM, nice work!