cli,signal: fix unknown value reporting

At this point we want the values enclosed between single quotes,
e.g. so that the fields are parsed easier by scripts.

This issue will not have any practical relevance as soon as the
key-value pair output support is merged.

cli/mmcli-modem-signal.c

@@ -140,7 +140,7 @@ print_signal_value (const gchar *prefix,
     if (value != MM_SIGNAL_UNKNOWN)
         g_print ("%s'%.2lf' %s\n", prefix, value, units);
     else
-        g_print ("%sn/a\n", prefix);
+        g_print ("%s'n/a'\n", prefix);
 }
 
 static void