lib: Use NULLSTR family of macros more

There is a family of convenient macros: NULLSTR, NULLSTR_EMPTY,
NULLSTR_STAR, NULLSTR_MINUS which hides ternary operator.

Generated using the following spatch (and its obvious variants):

  @@
  expression s;
  @@
    <+...
  - s ? s : "<null>"
  + NULLSTR(s)
    ...+>

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tools/virsh-checkpoint.c

@@ -455,7 +455,7 @@ cmdCheckpointInfo(vshControl *ctl,
                  _("unexpected problem querying checkpoint state"));
         return false;
     }
-    vshPrint(ctl, "%-15s %s\n", _("Parent:"), parent ? parent : "-");
+    vshPrint(ctl, "%-15s %s\n", _("Parent:"), NULLSTR_MINUS(parent));
 
     /* Children, Descendants.  */
     flags = 0;

tools/virsh-domain-monitor.c

@@ -712,11 +712,11 @@ cmdDomiflist(vshControl *ctl, const vshCmd *cmd)
         mac = virXPathString("string(./mac/@address)", ctxt);
 
         if (vshTableRowAppend(table,
-                              target ? target : "-",
+                              NULLSTR_MINUS(target),
                               type,
-                              source ? source : "-",
-                              model ? model : "-",
-                              mac ? mac : "-",
+                              NULLSTR_MINUS(source),
+                              NULLSTR_MINUS(model),
+                              NULLSTR_MINUS(mac),
                               NULL) < 0)
             return false;
     }

tools/virsh-nodedev.c

@@ -1233,7 +1233,7 @@ cmdNodeDeviceInfo(vshControl *ctl, const vshCmd *cmd)
 
     parent = virNodeDeviceGetParent(device);
     vshPrint(ctl, "%-15s %s\n", _("Name:"), virNodeDeviceGetName(device));
-    vshPrint(ctl, "%-15s %s\n", _("Parent:"), parent ? parent : "");
+    vshPrint(ctl, "%-15s %s\n", _("Parent:"), NULLSTR_EMPTY(parent));
     vshPrint(ctl, "%-15s %s\n", _("Active:"), virNodeDeviceIsActive(device) ?
              _("yes") : _("no"));
     vshPrint(ctl, "%-15s %s\n", _("Persistent:"),