[Variant] extend shredded null handling for arrays

[alamb]

@alamb -- we should track a follow-on item somewhere: Once we support array element shredding, we will have three different semantics to deal with, and this fix will need to expand from a bool to an enum:

When appending NULL:

	struct	value	typed_value
Variant	NULL	NULL	NULL
Object field	valid	NULL	NULL
Array element	valid	Variant::Null	NULL

Originally posted by @scovich in #8395 (comment)

[sdf-jkl]

Array element shredding is supported, and this is ready for someone to work on.