A fully unset (i.e. null IDProperty storage pointer) dynamic RNA
property would not be properly handled by RNA copy code path. This led
to having a type mismatch (because dynamic RNA booleans are stored in
integer IDProperies) betwen source and destination RNA properties.
Since the fix in 6adf5adb7f, such type mismatch will abort the copying
process.
This is fixed by updating the `PropertyRNAOrID` internal struct to also
store whether the given property is actually a dynamic RNA one with
IDProperty storage.
NOTE: Potential side-effects: That kind of change is very difficult to
fully analyse, but no side-effects from this fix are expected. The only
code which behavior is changed is `rna_ensure_property_realdata`, which
is only used by `RNA_property_copy` currently.
This change to `PropertyRNAOrID`/`rna_property_rna_or_id_get` _may_ be
useful in the future though, e.g. in liboverride codepath...
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