fix: pfsense backend (#703)

* Return empty string instead of "<nil>" when a genericjsonobject key doesn't exist.

* Fix pfsense backend

* Fix API request parameter names and types
* Refactor interface and peer creation to send the necessary parameters
* Automatically call apply when interfaces or peers are changed

Signed-off-by: Aram Akhavan <1147328+kaysond@users.noreply.github.com>

---------

Signed-off-by: Aram Akhavan <1147328+kaysond@users.noreply.github.com>

internal/lowlevel/mikrotik.go

@@ -57,6 +57,9 @@ type EmptyResponse struct{}
 
 func (JsonObject GenericJsonObject) GetString(key string) string {
 	if value, ok := JsonObject[key]; ok {
+		if value == nil {
+			return ""
+		}
 		if strValue, ok := value.(string); ok {
 			return strValue
 		} else {