[WPE][GTK] Flatpak check on host without flatpak throws a traceback

https://bugs.webkit.org/show_bug.cgi?id=257376

Reviewed by Michael Catanzaro.

* Tools/flatpak/flatpakutils.py:
(check_flatpak): Return early when flatpak command is not found.

Canonical link: https://commits.webkit.org/264583@main

Author: philn

Tools/flatpak/flatpakutils.py

@@ -196,7 +196,7 @@ def check_flatpak(verbose=True):
                                   " how to install it for your distribution at:\n"
                                   "    * https://flatpak.org/\n", required_version,
                                   sys.argv[0])
-            return ()
+        return ()
 
     def comparable_version(version):
         return tuple(map(int, (version.split("."))))