LibJS: Allow function calls with missing arguments

We were interpreting "undefined" as a variable lookup failure in some
cases and throwing a ReferenceError exception instead of treating it
as the valid value "undefined".

This patch wraps the result of variable lookup in Optional<>, which
allows us to only throw ReferenceError when lookup actually fails.

Libraries/LibJS/Tests/function-missing-arg.js

@@ -0,0 +1,13 @@
+function assert(x) { if (!x) throw 1; }
+
+function foo(a, b) { return a + b; }
+
+try {
+    assert(isNaN(foo()) === true);
+    assert(isNaN(foo(1)) === true);
+    assert(foo(2, 3) === 5);
+    assert(foo(2, 3, 4) === 5);
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}