fix(ServerRpc): fixing error message when client is not set

if behaviour isn't spawned it wont have a Client so will give null reference exception

Assets/Mirage/Runtime/NetworkBehaviour.cs

@@ -248,7 +248,7 @@ private void ValidateServerRpc(Type invokeClass, string cmdName, bool requireAut
             // this was in Weaver before
             // NOTE: we could remove this later to allow calling Cmds on Server
             //       to avoid Wrapper functions. a lot of people requested this.
-            if (!Client.Active)
+            if (Client == null || !Client.Active)
             {
                 throw new InvalidOperationException($"ServerRpc Function {cmdName} called on server without an active client.");
             }