### What changes were proposed in this pull request?

This is a followup of apache#27627 to fix the remaining issues. There are 2 issues fixed in this PR:
1. `UnsafeRow.setDecimal` can set an overflowed decimal and causes an error when reading it. The expected behavior is to return null.
2. The update/merge expression for decimal type in `Sum` is wrong. We shouldn't turn the `sum` value back to 0 after it becomes null due to overflow. This issue was hidden because:
2.1 for hash aggregate, the buffer is unsafe row. Due to the first bug, we fail when overflow happens, so there is no chance to mistakenly turn null back to 0.
2.2 for sort-based aggregate, the buffer is generic row. The decimal can overflow (the Decimal class has unlimited precision) and we don't have the null problem.

If we only fix the first bug, then the second bug is exposed and test fails. If we only fix the second bug, there is no way to test it. This PR fixes these 2 bugs together.

### Why are the changes needed?

Fix issues during decimal sum when overflow happens

### Does this PR introduce _any_ user-facing change?

Yes. Now decimal sum can return null correctly for overflow under non-ansi mode.

### How was this patch tested?

new test and updated test

Closes apache#29026 from cloud-fan/decimal.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

fix error: java.lang.IllegalArgumentException: Can not interpolate java.lang.Boolean into code block.

fix ci error