tools: use NULLSTR_MINUS

Use the newly introduced macro in the few places that open-code it.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

tools/virsh-domain-monitor.c

@@ -643,11 +643,11 @@ cmdDomblklist(vshControl *ctl, const vshCmd *cmd)
                                 "|./source/@volume)", ctxt);
         if (details) {
             if (vshTableRowAppend(table, type, device, target,
-                                  source ? source : "-", NULL) < 0)
+                                  NULLSTR_MINUS(source), NULL) < 0)
                 goto cleanup;
         } else {
             if (vshTableRowAppend(table, target,
-                                  source ? source : "-", NULL) < 0)
+                                  NULLSTR_MINUS(source), NULL) < 0)
                 goto cleanup;
         }
 

tools/virsh-snapshot.c

@@ -945,7 +945,7 @@ cmdSnapshotInfo(vshControl *ctl, const vshCmd *cmd)
     /* Since we already have the XML, there's no need to call
      * virDomainSnapshotGetParent */
     parent = virXPathString("string(/domainsnapshot/parent/name)", ctxt);
-    vshPrint(ctl, "%-15s %s\n", _("Parent:"), parent ? parent : "-");
+    vshPrint(ctl, "%-15s %s\n", _("Parent:"), NULLSTR_MINUS(parent));
 
     /* Children, Descendants.  After this point, the fallback to
      * compute children is too expensive, so we gracefully quit if the