Backport of objchange: fix ProposedNew from null objects into v1.0

[teamterraform]

Backport

This PR is auto-generated from #29701 to be assessed for backporting due to the inclusion of the label 1.0-backport.

The below text is copied from the body of the original PR.

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The codepath for AllAttributesNull was not correct for any nested object
types with collections, and should create single null values for the
correct NestingMode rather than a single object with null attributes.
Since there is no reason to descend into nested object types to create
null values, we can drop the AllAttributesNull function altogether and
create null values as needed during ProposedNew.

The corresponding AllBlockAttributesNull was only called internally in 1
location, and simply delegated to schema.EmptyValue. We can reduce the
package surface area by dropping that function too and calling
EmptyValue directly.

Fixes #29697

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