Typically, when a group is specified, channels will be specified as
well. However, if due to error or mistake, the user forgets to specify
ANY channels for a group, we should not generate that group's
information.
This means that no channels are specified (which maps the remote
master to local slave). In addition, the `Group <gName>` block (which
brings the separate channels together) is also not generated.
Another thing to consider is that a user might specify a group and a
channel, but perform no additional configuration of the channel.
In a configuration, this would be realized by
`accounts.email.accounts.<aName>.mbsync.groups.<gName>.channels.<cName>;`
This creates the channel with the name `<cName>` and the
`masterPattern`, `slavePattern`, and `patterns` fields use their defaults.
By definitions set within mbsync, these defaults actually specify that
the remote master's `INBOX` mail directory is synchronized to the
local slave's `INBOX` directory.
So, if there is a channel that has no fields specified, then we DO
want to generate its configuration. But if there is a group that has
no channels, then we do NOT generate it.
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