AK: Treat empty string as invalid JSON

Previously we would treat the empty string as `null`. This caused
JavaScript like this to fail:
```js
var object = {};
try {
    object = JSON.parse("");
} catch {}
var array = object.array || [];
```
Since `JSON.parse("")` returned null instead of throwing, it would set
`object` to null and then try and use it instead of using the default
backup value.

Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.parse.js

@@ -29,6 +29,7 @@ test("syntax errors", () => {
         "[1,2,3, ]",
         '{ "foo": "bar",}',
         '{ "foo": "bar", }',
+        "",
     ].forEach(test => {
         expect(() => {
             JSON.parse(test);