all: Adjust UseStateForUnknown plan modifiers to preserve known null state values

[austinvalle]

Related Issue

Closes #1117

Description

This PR adjusts all of the UseStateForUnknown plan modifiers to also preserve null state values (which are technically known). The original logic was excluding all null state values although we believe the intended behavior was to skip when creating resources (which should be checking the resource instance state for null, not the specific state value).

Note

One of the impacts of fixing this bug would be any attributes in existing resource implementations that are relying on null values being set to unknown during update plans, which would now start to preserve the prior state (keeping null), so if the provider is not preserving those prior state values during apply, it could cause data consistency issues if that unknown marking was being relied on.

Off the top of my head, I'm not sure why you'd use UseStateForUnknown in this way, since the goal of this plan modifier has always been to denote an attribute that doesn't change during apply on updates. Fixing this bug doesn't change that goal, it just makes it more precise.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No

[austinvalle]

The changes to this test and internal/fwserver/block_plan_modification_test.go were just to ensure the intended behavior was still tested, as the original tests did not actually set up the req.State value properly 😄