### What changes were proposed in this pull request?
Replaced the `agg(if (('gid = 1)) 'cat1 else null)` pattern in `RewriteDistinctAggregates` with `agg('cat1) FILTER (WHERE 'gid = 1)`
### Why are the changes needed?
For aggregate functions, that do not ignore NULL values (`First`, `Last` or `UDAF`s) the current approach can return wrong results.
In the added UT there are no nulls in the input `testData`. The query returned `Row(0, 1, 0, 51, 100)` before this PR.
### Does this PR introduce _any_ user-facing change?
Bugfix
### How was this patch tested?
UT
Closes#31983 from tanelk/SPARK-34882_distinct_agg_filter.
Lead-authored-by: Tanel Kiis <tanel.kiis@gmail.com>
Co-authored-by: tanel.kiis@gmail.com <tanel.kiis@gmail.com>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
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