Make sure modContext config is "prepared" before using makeUrl()

### What does it do ?

Calls `modContext->prepare()` before using `modContext->makeUrl()` if `modContext->config` is `null`

### Why is it needed ?

If you iterate modContext & try to use `makeUrl`, the generated URL is wrong since contexts aren't prepared (unless you manually call the method on each object).

This aims to ease the usage of `modContext->makeUrl`