add possibility to only sync Slack handles w/o managing channel topics

[m3co-code]

Hey @timoreimann :-)

Thanks for the nice lib, still in active use ofc.

Right now it doesn't seem to be possible to only leverage the user handles for the oncall groups, w/o the tool to also sync the channel topic.

The guard for a slack channel being present when executing the sync in https://github.com/timoreimann/pdsync/blob/master/syncer.go#L132 is the culprit imho. To make it through the guard a channel needs to be specified and once a channel is configured the template must not be empty.

I might come around the next days / few weeks to (re)add the feature.

[timoreimann]

Sounds totally reasonable. Happy to review any PRs. 🙂