Handle case where `bindings` are removed from vn resource

[chrismarget-j]

I think that a user creating a VN with bindings, then deleting those bindings (possible with 5.x) will find that the bindings do not go away.

This is consistent with terraform's "I don't care about that value" approach, but puts us in a tough position when no bindings are found during Read(). Do we return bindings = null or bindings = {}?

Possible workarounds:

• continue to disallow bindings = [] and store a hint about the prior binding configuration in the state (private state?)
• allow both null and {} bindings, and create a custom type which treats these as semantically equal. - possible strange feeling behavior where data source returns different style than used in resource.
• disallow VN bindings with Apstra 5.x - force users to use new vn_binding resource (does not currently exist)

[chrismarget-j]

It looks like the current strategy (note presence of bindings in the state) solves this problem:

https://github.com/Juniper/terraform-provider-apstra/blob/ffca27533382aff6c48229444fb61f560e0601f4/apstra/resource_datacenter_virtual_network.go#L294C1-L304C1