UrlResolverImpl should work with context = null

[imeszaros]

Pac4j's SAML2Client does a weird thing at initialization which will cause an NPE to be thrown because jooby-pac4j's UrlResolverImpl is not prepared for the case that the argument context can be null.

I have not the slightest idea why is it like this but I could only overcome this issue by plugging in a custom implementation of UrlResolver that itself builds the full URL purely from the applications Config instead of using the context.