[frama-c-script] fix flamegraph command

Adding option `-u` to frama-c-script led to an "unbound variable" error when
running `frama-c-script flamegraph`.

bin/frama-c-script

@@ -195,7 +195,7 @@ flamegraph() {
   </body>
 </html>
 EOF
-    if [ -z "$NOGUI" ]; then
+    if [ -z "${NOGUI-}" ]; then
         open_file "$out_html"
     fi
 }