[Vectorized][Enhancement] fix some bug & improve some code #7713

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Description

1. fix core dump when read empty block
2. fix deadlock on volap_scan_node
3. improve agg reader hash_map use
4. improve reader agg function register
5. reduce run parallel on run-be-ut.sh to avoid oom at github workflow

Solution

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Are you willing to submit PR?

• Yes I am willing to submit a PR!

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