admin: Allow passing NULL to virLogSetOutputs

Along with an empty string, it should also be possible for users to pass
NULL to the public APIs which in turn would trigger a routine(future
work) responsible for defining an appropriate default logging output
given the current circumstances.

Signed-off-by: Erik Skultety <eskultet@redhat.com>

daemon/libvirtd.c

@@ -693,7 +693,7 @@ daemonSetupLogging(struct daemonConfig *config,
     if (virLogGetNbFilters() == 0)
         virLogSetFilters(config->log_filters);
 
-    if (config->log_outputs && virLogGetNbOutputs() == 0)
+    if (virLogGetNbOutputs() == 0)
         virLogSetOutputs(config->log_outputs);
 
     /*