network_acl: fix state not optional

[positiveEV]

This PR makes the state attribute in network_acl optional, as documented at

terraform-provider-incus/docs/resources/network_acl.md

Line 92 in c458c3e

* `state` - *Optional* - State of the rule (enabled, disabled or logged), defaulting to enabled if not specified

[maveonair]

@positiveEV please sign off your commit, and have a look at the tests which are failing due to this change.

[positiveEV]

I found the issue.
When tofu is planning, #385 sets null attributes as empty strings. This triggers a bug that I found in the plugin framework (hashicorp/terraform-plugin-framework#1256) when using default values for attributes in nested blocks, which is why the state is set to its default value instead of the configured one.

A workaround could be to set all attributes that we do not use in the egress and ingress blocks as empty string, so we would not encounter hashicorp/terraform-plugin-framework#1256. But it is obviously not desirable.

It means that we should wait for #385 to be resolved before being able to merge this PR.
So for now, I think it is probably best to just fix the doc.