libvirt: correctly print out URI returned from probing

Turning a NULL URI instead the empty string is very misleading when
reading the debug logs as the distinction between the two is
functionally important.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/libvirt.c

@@ -886,7 +886,7 @@ virConnectOpenInternal(const char *name,
                         goto failed;
                     VIR_DEBUG("%s driver URI probe returned '%s'",
                               virConnectDriverTab[i]->hypervisorDriver->name,
-                              NULLSTR_EMPTY(uristr));
+                              NULLSTR(uristr));
                 }
             }
         }