Update spec for conversion of uninitialized Value proto to null.

[JimLarson]

The google.protobuf.Value proto is more useful if its default value has a meaningful CEL value. Converting it to null matches the spirit of the various *Value wrapper types, as well as the spirit of null itself.

[JimLarson]

Note that this creates an ambiguity in reading an unset field vs one set to null. This can be semantically significant, as the Value proto definition says "absence of any variant indicates an error". However, the ambiguity can be resolved by testing with has().