Fix running scripts without arguments

Fixes #9660

because when there are no arguments, `argument` value is null
and the check is throwing an exception.

Remove the check because it doesn't cause a problem to the process
if null is passed...

GitUI/Script/ScriptRunner.cs

@@ -91,8 +91,6 @@ private static CommandStatus RunScriptInternal(IWin32Window owner, IGitModule mo
                     new ExternalOperationException(scriptInfo.Command, arguments, module.WorkingDir));
             }
 
-            Validates.NotNull(argument);
-
             string command = OverrideCommandWhenNecessary(originalCommand);
             command = ExpandCommandVariables(command, module);