Minor: Empty list returned by poll() destroys the source task when assert is enabled

[chia7712]

I noticed this issue when running the embedded connectors. There is a assert in SourceRecordWriteCounter.

        public SourceRecordWriteCounter(int batchSize, SourceTaskMetricsGroup metricsGroup) {
            assert batchSize > 0;   // this one
            assert metricsGroup != null;
            this.batchSize = batchSize;
            counter = batchSize;
            this.metricsGroup = metricsGroup;
        }

Hence, the empty list will cease the source task. By contrast, empty list is not a issue in production since the assert is usually disabled in production. It seems to me making the behavior consistent is necessary. A simple fix is to remove the assert. Or empty list should be handled as null list. I prefer later since both of them mean "no data can be processed".

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[kkonstantine]

Closing this PR in favor of #7491 which seems to have fixed the issue.