[HUDI-8411] Fix unmerged reader to skip delete blocks and avoid NPE

[jonvex]

Change Logs

reader context had default implementation of get engine delete representation as null. The unmerged reader does not have handling for that. When compared with the other unmerged reader implementations they never process delete blocks, so the unmerged buffer is now updated to skip delete blocks

Impact

Spark npe is avoided and should be a slight perf boost. Also 1 less reader context method which is good.

Risk level (write none, low medium or high below)

none

Documentation Update

N/A

Contributor's checklist

• [ ] Read through contributor's guide
• [ ] Change Logs and Impact were stated clearly
• [ ] Adequate tests were added if applicable
• [ ] CI passed

[hudi-bot]

CI report:

• 6221ed0a430d211048b122003f48ca0e5472a123 Azure: SUCCESS

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