[core] Fix ETag returns for async queries
[core] Fix ETag returns for async queries

If an async query had an ETag match, a NULL feed was returned, but the
async query code didn't handle this properly.

Distrotech/libgdata,Distrotech/libgdata,GNOME/libgdata,Pecisk/libgdata-gtasks,Pecisk/libgdata,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata-gtasks,Pecisk/libgdata,Pecisk/libgdata,pwithnall/libgdata,pwithnall/libgdata