improvements

Pilz.Net/Api/ApiClient.cs

@@ -1,6 +1,4 @@
 using Castle.Core.Logging;
-using Pilz.Extensions.Collections;
-using System.Text.Encodings.Web;
 using System.Web;
 
 namespace Pilz.Net.Api;
@@ -102,9 +100,9 @@ public class ApiClient(string apiUrl) : IApiClient
         }));
     }
 
-    protected virtual string ConvertParameter(object @param)
+    protected virtual string? ConvertParameter(object? @param)
     {
-        return (string)Convert.ChangeType(@param, typeof(string));
+        return (string?)Convert.ChangeType(@param, typeof(string));
     }
 
     protected virtual string? EncodeAuthKey()