Fix piping not using array for empty unnamed args

- Lenny said if `splitUnnamedArgument` is enabled, the callback should always receive an array.
- It is intended that if no value was provided, it'll get an array with an empty string. Because.. if no argument was provided for a non-split arg, it'll receive an empty string too.

public/scripts/slash-commands/SlashCommandClosure.js

@@ -508,6 +508,14 @@ export class SlashCommandClosure {
                 return v;
             });
         }
+
+        value ??= '';
+
+        // Make sure that if unnamed args are split, it should always return an array
+        if (executor.command.splitUnnamedArgument && !Array.isArray(value)) {
+            value = [value];
+        }
+
         return value;
     }