Fix missing completion when no buckets are in checkpoint

[simolus3]

For clients, a full sync iteration generally looks like this:

1. They receive a checkpoint message describing the target state of each bucket.
2. For each bucket, data lines are sent for data.
3. Finally, a checkpoint_complete line is sent and allows clients to make the lines they've received public if it matches the target state from earlier.

After the changes to the sync service for bucket priorities and partial completions, we also send a partial_checkpoint_complete line as part of step 2 after sending all data lines in a specific priority (except for the lowest priority, which results in the traditional checkpoint_complete line).
These changes introduced a regression observable when performing a sync iteration with no changed buckets (which can happen when only write checkpoints are changed). Since there are no buckets and no priorities, there is no lowest priority for which a checkpoint_complete line would be sent. This breaks clients, which only consider the sync complete after receiving that line.

Fix this by ensuring we always send a checkpoint_complete after a finished sync operation.

[changeset-bot]

🦋 Changeset detected

Latest commit: cbc4de8

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@powersync/service-core	Patch
@powersync/service-core-tests	Patch
@powersync/service-module-mongodb-storage	Patch
@powersync/service-module-mongodb	Patch
@powersync/service-module-mysql	Patch
@powersync/service-module-postgres-storage	Patch
@powersync/service-module-postgres	Patch
@powersync/service-image	Patch
test-client	Patch

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[stevensJourney]

Verified this fixes the issue on my end. Thanks!