Handle the situation when user_getfield('cla') returns None
This can happen if we don't query the OIDC provider correctly (with the
correct scopes) and in that situation fedocal ends up crashing.
With this commit, it will assume the list of signed agreements is empty
and the application will keep on going.
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>