LibJS: Change normal_completion() parameter to Optional<Value>

The Completion constructor `VERIFY()`s that the passed argument is not
an empty Value, so normal_completion({}) would crash (although it's
currently not being used anywhere).
We want to pass an empty Optional<Value> instead.

Userland/Libraries/LibJS/Runtime/Completion.h

@@ -157,7 +157,7 @@ public:
 };
 
 // 6.2.3.2 NormalCompletion ( value ), https://tc39.es/ecma262/#sec-normalcompletion
-inline Completion normal_completion(Value value)
+inline Completion normal_completion(Optional<Value> value)
 {
     return { Completion::Type::Normal, value, {} };
 }