application: Fix working-directory when cwd is NULL

When working-directory is specified, a path is built using
g_file_new_for_commandline_arg_and_cwd() using the current cwd.
That's redundant compared to using g_file_new_for_commandline_arg().
This is a problem because if cwd is NULL, you'll never get a path
(this is the case when calling from dbus).
3 jobs for working_directory in 2 minutes and 39 seconds
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