LibJS: Implicitly break for..in loop if the RHS result is nullish

This implements the missing step 6a of 14.7.5.6 ForIn/OfHeadEvaluation:

    a. If exprValue is undefined or null, then
        i. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.

In other words, this should just do nothing instead of throwing during
the undefined to object coercion:

    for (const x in undefined);

Userland/Libraries/LibJS/AST.cpp

@@ -477,6 +477,8 @@ Value ForInStatement::execute(Interpreter& interpreter, GlobalObject& global_obj
     auto rhs_result = m_rhs->execute(interpreter, global_object);
     if (interpreter.exception())
         return {};
+    if (rhs_result.is_nullish())
+        return {};
     auto* object = rhs_result.to_object(global_object);
     while (object) {
         auto property_names = object->get_enumerable_own_property_names(Object::PropertyKind::Key);

Userland/Libraries/LibJS/Tests/loops/for-in-basic.js

@@ -38,6 +38,12 @@ test("iterate through object", () => {
     expect(a).toEqual(["a", "b", "c"]);
 });
 
+test("iterate through undefined", () => {
+    for (const property in undefined) {
+        expect.fail();
+    }
+});
+
 test("use already-declared variable", () => {
     var property;
     for (property in "abc");