mct_app_filter_is_appinfo_allowed fails when desktop entry is missing Exec key

mct_app_filter_is_appinfo_allowed() contains:

abs_path = g_find_program_in_path (g_app_info_get_executable (app_info));

When a GDesktopAppInfo has no Exec key, g_app_info_get_executable() is NULL (even though it's not documented as nullable). That results in log messages like this from gnome-shell:

gnome-shell[2410]: g_find_program_for_path: assertion 'program != NULL' failed

See https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7313