[dav] Ensure we have a valid username (#582373)
[dav] Ensure we have a valid username (#582373)

If g_mount_source_ask_password didn't fill in a new username fall
back to the one we pre-filled it with.
Patch by Frederic Peters <fpeters@0d.be>

cpatulea/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,halfline/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,cpatulea/gvfs,thiblahute/gvfs,gicmo/gvfs,philipl/gvfs,halfline/gvfs,philipl/gvfs,halfline/gvfs,thiblahute/gvfs,philipl/gvfs,xkahn/gvfs-cmis