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.

AK/JsonValue.cpp

@@ -236,8 +236,6 @@ void JsonValue::clear()
 #ifndef KERNEL
 ErrorOr<JsonValue> JsonValue::from_string(StringView input)
 {
-    if (input.is_empty())
-        return JsonValue();
     return JsonParser(input).parse();
 }
 #endif