virsh: allow empty targets in cmdDomFSInfo

Ever since the introduction of the guest-get-fsinfo command
in QEMU commit 46d4c572 qga/qapi-schema.json says that
the 'disks' array can possibly be empty. For example when getting
the target list is unsupported:
https://bugzilla.redhat.com/show_bug.cgi?id=1567041

Pass an empty string instead of NULL to vshTableRowAppend to prevent
a mismatched column number.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

tools/virsh-domain.c

@@ -13978,7 +13978,7 @@ cmdDomFSInfo(vshControl *ctl, const vshCmd *cmd)
                                   info[i]->mountpoint,
                                   info[i]->name,
                                   info[i]->fstype,
-                                  targets,
+                                  targets ? targets : "",
                                   NULL) < 0)
                 goto cleanup;
         }