bench: arrow batch benchmarks broken

[cucaroach]

After this commit (0d580ad) we crash on nil pointers.

[yuzefovich]

Oh yeah, that makes sense. I think we simply want to do something like

diff --git a/pkg/col/colserde/arrowbatchconverter_test.go b/pkg/col/colserde/arrowbatchconverter_test.go
index 06a8cee258..aa3e017be4 100644
--- a/pkg/col/colserde/arrowbatchconverter_test.go
+++ b/pkg/col/colserde/arrowbatchconverter_test.go
@@ -224,15 +224,19 @@ func BenchmarkArrowBatchConverter(b *testing.B) {
                for _, nullFraction := range nullFractions {
                        setNullFraction(batch, nullFraction)
                        data, err := c.BatchToArrow(batch)
+                       dataCopy := make([]*array.Data, len(data))
                        require.NoError(b, err)
                        testPrefix := fmt.Sprintf("%s/nullFraction=%0.2f", typ.String(), nullFraction)
                        result := testAllocator.NewMemBatchWithMaxCapacity([]*types.T{typ})
                        b.Run(testPrefix+"/ArrowToBatch", func(b *testing.B) {
                                b.SetBytes(numBytes[typIdx])
                                for i := 0; i < b.N; i++ {
+                                       // Since ArrowToBatch eagerly nils things out, we have to
+                                       // make a shallow copy each time.
+                                       copy(dataCopy, data)
                                        // Using require.NoError here causes large enough allocations to
                                        // affect the result.
-                                       if err := c.ArrowToBatch(data, batch.Length(), result); err != nil {
+                                       if err := c.ArrowToBatch(dataCopy, batch.Length(), result); err != nil {
                                                b.Fatal(err)
                                        }
                                        if result.Width() != 1 {

[cucaroach]

I went in a different direction, I think that fix would make us look bad performance wise.