As a follow up of MDL-74398, the instance->get_group_id() was supposed to return null when course group not enabled.

However, if we do that this impacts also the API and as close to the release date this is likely risky to make further change. A workaround has been found for MDL-74398 but we still need to work on the meaning of group_id for a meeting:

• If we allow get_group_id to return null we will have to check for its value before including it into any API call and change the API to make the groupid optional.
• The meeting name would also not contain [0] as group id =0. This might be an issue later if we just enable grouping in the same course later on. This have to be checked before we change this.
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