fix: netbox_permission plan constraint diff

Every run of the `netbox_permission` resource when there was no
constraint set would output the following diff:
```
netbox_permission.ipaddress_rw_permission will be updated in-place
  ~ resource "netbox_permission" "ipaddress_rw_permission" {
      - constraints  = "null" -> null
        id           = "3"
        name         = "ipaddress_rw_permissions"
```

There are inherently two issues that cause the problem. We cannot null
the field, because of the issue fixed here:
fbreckle/go-netbox#30

Second, want the constraint field to be set to an empty string when
a read of constraint is empty. So we check if nil, and if so we
set constraint to "", and then return the function. Otherwise, we
Marshal it to JSON to fill the tf state.

netbox/resource_netbox_permission.go

@@ -77,7 +77,6 @@ func resourceNetboxPermission() *schema.Resource {
 				Description: "A JSON string of an arbitrary filter used to limit the granted action(s) to a specific subset of objects. " +
 					"For more information on correct syntax, see https://docs.netbox.dev/en/stable/administration/permissions/#constraints ",
 				Optional:     true,
-				Default:      nil,
 				ValidateFunc: validation.StringIsJSON,
 			},
 		},
@@ -165,6 +164,11 @@ func resourceNetboxPermissionRead(d *schema.ResourceData, m interface{}) error {
 
 	d.Set("actions", res.GetPayload().Actions)
 
+	if res.GetPayload().Constraints == nil {
+		d.Set("constraints", "")
+		return nil
+	}
+
 	b, err := json.Marshal(res.GetPayload().Constraints)
 	if err != nil {
 		return err