Draft: Mark g_app_info_get_executable() return value as nullable

Open Sebastian Dröge requested to merge sdroege/glib:appinfo-get-executable-nullable into main

It can't return NULL on macOS, will return NULL on other UNIXes if the .desktop file has no Exec= line and can easily return NULL on Windows under various conditions.

Also it's clearly intended that this can be NULL from the check in gdesktopappinfo.c:expand_application_parameters() at the top (checking for an empty Exec= line).

Fixes #2974

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