core: Don't panic if ApplyResourceChange returns invalid new value
core: Don't panic if ApplyResourceChange returns invalid new value

The provider is allowed to return a partial result if it also includes
error diagnostics. Real providers still return at least a null value in
that case due to the RPC format, but test mocks are often more sloppy.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk