fix nil-relaxing behavior

[tosuke]

This PR addresses the behavior of the NilRelaxed plan modifier.

The Terraform Plugin SDK silently updates the state for optional attributes, while the Plugin Framework does not. To bridge this gap, we introduced the NilRelaxed plan modifier in #248. However, as discussed in hashicorp/terraform-plugin-framework#510, we discovered that it cannot completely replicate the SDK's behavior.

The SDK handles attributes with null or zero values in the following way:

1. Sets them to null during the first apply
2. Converts null values to type-appropriate zero values during refresh operations

When examining the relationship between configuration values and state values, the SDK considers these scenarios valid:

1. Configuration is null, state is null
2. Configuration is zero value, state is null
3. Configuration is null, state is zero value
4. Configuration is zero value, state is zero value

Of these scenarios, cases 2 and 3 generate non-empty plans in the Framework due to mismatches between configuration and state. The NilRelaxed plan modifier aimed to prevent unnecessary plan generation in these cases by adopting the previous state's value.

However, due to constraints in Terraform's type system (go-cty), we discovered that case 2 cannot be handled - specifically, we cannot revert a plan that converts a zero value back to null without raising an error. Since case 2 only occurs during the first apply, we've decided to accept these plan generations as expected behavior.

Output from acceptance testing:

$ make testacc TESTS=TestAccXXX

...

[tosuke]

Handling unknown values to emulate Optional behavior