Forbid null schemes and realms in Challenge.
Forbid null schemes and realms in Challenge.

These were never null in practice.

square/okhttp,jrodbx/okhttp,cketti/okhttp,ansman/okhttp,tiarebalbi/okhttp,cmzy/okhttp,ze-pequeno/okhttp,ze-pequeno/okhttp,ansman/okhttp,cmzy/okhttp,tiarebalbi/okhttp,yschimke/okhttp,jrodbx/okhttp,yschimke/okhttp,ansman/okhttp,tiarebalbi/okhttp,cketti/okhttp,ze-pequeno/okhttp,square/okhttp,yschimke/okhttp,jrodbx/okhttp,cketti/okhttp,square/okhttp