When the gantt chart service was first implemented, the read_token()
function would attempt to read from the default token file if the token
argument was missing.
Now it's necessary to call a separate function to look in the default
token file, so if the token argument is not provided, expressly call
get_token_from_default_dir().
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32637>
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