Ok, now that we have established that the user's encoding is supposed to
be mac-celtic, I think I understand the problem: there simply isn't any
IANA charset name for the mac-celtic encoding, so
CFStringConvertEncodingToIANACharSetName doesn't return any.

If we want to support these systems, I think we need to add another
switch case for mac-celtic. That alone won't be sufficient, as we then
also need an implementation of the encoding, i.e. change Tools/unicode
to preserve mac-celtic.

To better detect that case in the future, it might be useful to return
mac-unsupported as the script name if it isn't in the switch statement
and doesn't have a IANA name, and then alias mac-unknown to ASCII.