issue: #43980
Fix panic issue caused by incorrect nullable field merging logic when
upsert converts to insert operation on empty tables.
- Add AppendFieldDataWithNullData to handle nullable field merging
- Fix existing data merge with skipAppendNullData=false
- Fix insert data merge with skipAppendNullData=true
- Add unit tests for nullable field data appending scenarios
Signed-off-by: Wei Liu <wei.liu@zilliz.com>