Avoid to generate empty split in source emuerator

[luoyuxia]

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• I searched in the issues and found nothing similar.

Description

Currently, generateSplitForPrimaryKeyTableBucket and generateSplitForLogTableBucket will generate splits even though the split is empty (the latestBucketOffset is 0), and assign to reader. We can improve it, when latestBucketOffset is 0, we don't need to generate the split.

Willingness to contribute

• I'm willing to submit a PR!

[LiebingYu]

@luoyuxia I can take it.