[VL] Ignore null values when copy velox string buffer to arrow

[wankunde]

What changes are proposed in this pull request?

Bug fix for VeloxHashShuffleWriter.
Description:

• VeloxHashShuffleWriter::write() method will try to convert the input velox flat vectors into Arrow buffer.
• For StringView vectors, VeloxHashShuffleWriter calculates the total size of all input StringViews, and initializes the Arrow buffer accordingly.
• For null values, we do not actually need to copy the dirty values into Arrow buffers, thereby saving execution memory.

Test logs in our productions:

I20260415 17:50:28.520978  5474 VeloxHashShuffleWriter.cc:156] collectFlatVectorBufferStringView for VARCHAR
I20260415 17:50:28.521070  5474 VeloxHashShuffleWriter.cc:110] DEBUG, lengthBufferSize = 40, flatVector->size() = 10
I20260415 17:50:28.521078  5474 VeloxHashShuffleWriter.cc:114] DEBUG flatVector = [FLAT VARCHAR: 10 elements, 9 nulls] data: 0: null
1: null
2: null
3: 114535255
4: null
5: null
6: null
7: null
8: null
I20260415 17:50:28.521092  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 0, isNull = true, rawValues[i].size() = 1957213616, length = 0,  offset = 0
I20260415 17:50:28.521100  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 1, isNull = true, rawValues[i].size() = 2588080704, length = 0,  offset = 0
I20260415 17:50:28.521106  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 2, isNull = true, rawValues[i].size() = 2706762760, length = 0,  offset = 0
I20260415 17:50:28.521111  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 3, isNull = false, rawValues[i].size() = 9, length = 9,  offset = 9
I20260415 17:50:28.521117  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 4, isNull = true, rawValues[i].size() = 0, length = 0,  offset = 9
I20260415 17:50:28.521122  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 5, isNull = true, rawValues[i].size() = 1, length = 0,  offset = 9
I20260415 17:50:28.521127  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 6, isNull = true, rawValues[i].size() = 2832791872, length = 0,  offset = 9
I20260415 17:50:28.521132  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 7, isNull = true, rawValues[i].size() = 2588222376, length = 0,  offset = 9
I20260415 17:50:28.521138  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 8, isNull = true, rawValues[i].size() = 0, length = 0,  offset = 9
I20260415 17:50:28.521143  5474 VeloxHashShuffleWriter.cc:120] DEBUG flatVector dump, i = 9, isNull = true, rawValues[i].size() = 0, length = 0,  offset = 9

How was this patch tested?

Tested in out production environment.

Was this patch authored or co-authored using generative AI tooling?

No

[wankunde]

Retest this please

[jinchengchenghh]

Thanks!

[zhouyuan]

@wankunde thanks for the fix, would you please also add a unit test to guard on this?