For Structures the rules are quite simple and clearly described in "Table 38 – JSON Encoding Rules for Structures":
Field Value Reversible Non-Reversible
NULL Omitted JSON null
Default Value Omitted Default Value

It seems the intention is to apply these rules also to built-in types which are encoded as JSON objects, as many of these state similar rules for their fields. However these rules differ in details from the above rules, e.g. the DataValue states "If a field has a null or DefaultValue it is omitted" without distinguishing between reversible and non-reversible encoding which means it must be omitted in both encodings.

So if the intention is to use the Structure rules for the built-in types this should be explicitly stated in the General section and the rules stated for individual built-in types should be removed unless the built-in type actually deviates from the general rules.