ImportState: Fix panic when identity attribute is null

[austinvalle]

This PR introduces a change to import configuration building of identities to account for the scenario where an OptionalForImport attribute.

It's technically valid for any identity attribute to be null, regardless of the behavior field set, but if a provider developer wants to test import via identity for a different scenario, where the attribute is provided in import, they can always create a custom config.

---

Test failure without the change:

--- FAIL: TestImportBlock_WithResourceIdentity_NullAttribute (1.36s)
panic: unexpected type <nil> for identity value "value_we_dont_always_need" [recovered]
	panic: unexpected type <nil> for identity value "value_we_dont_always_need"

[bbasata]