using AuthenticationMethod.EXTERNAL and an AccessProvider results in NullPointerException

[pdowler]

The url argument to ExternalAuthenticationCommandImpl constructor is null and passed to superclass, resulting in an attempt to create an HttpGet that throws the NullPointerException.

Simple fix: ExternalAuthenticationCommandImpl can override HttpGet createRequest(String url) and return a null. This is sufficient to allow a custom AccessProvider to work.

I have tested that this fix works, but there is no ExternalAuthenticationCommandImplTest to modify.

Ready to create a pull request with the simple fix if this looks OK or if review happens with the PR.