Remove null values from cache to improve Onyx read speed #553

In #411 we changed code in Onyx to persist null values in cache, in order for the user to know, that a value has been accessed before but there's no value.

This fixed an issue here but apperantly isn't the perfect solution, since it requires us to remove null values on read, as implemented in #519.

This PR removes null values from cache and removes calls to removeNestedNullValues on read. Instead, null values are only going to be removed on write (e.g. Onyx.merge/Onyx.set).

This should speed up read speeds by a lot, since it prevents the problem described in https://expensify.slack.com/archives/C05LX9D6E07/p1716395008375049?thread_ts=1716303470.244179&cid=C05LX9D6E07