LibJS: Use array-like size for IndexedProperties::is_empty()

Some things, like (the non-generic version of) Array.prototype.pop(),
check is_empty() to determine whether an action, like removing elements,
can be performed. We need to know the array-like size for that, not the
size of the underlying storage, which can be different - and is not
something IndexedProperties should expose so I removed its size().

Fixes #3948.

Libraries/LibJS/Tests/builtins/Array/Array.prototype.pop.js

@@ -7,6 +7,12 @@ describe("normal behavior", () => {
         var a = [1, 2, 3];
         expect(a.pop()).toBe(3);
         expect(a).toEqual([1, 2]);
+        expect(a.pop()).toBe(2);
+        expect(a).toEqual([1]);
+        expect(a.pop()).toBe(1);
+        expect(a).toEqual([]);
+        expect(a.pop()).toBeUndefined();
+        expect(a).toEqual([]);
     });
 
     test("empty array", () => {