fix(arrow/array): ignore zero-count union appends

[fallintoplace]

Rationale for this change

DenseUnionBuilder.AppendNulls(0) and AppendEmptyValues(0) still append a child value even though they add no union entries. That hidden mutation shifts the offset of the next real value and leaves inaccessible child data behind.

What changes are included in this PR?

• Make non-positive bulk null and empty appends no-ops.
• Preserve the existing shared-child optimization for positive counts.

Are these changes tested?

Yes. The regression test verifies that both zero-count calls leave the union and child builders empty, then confirms the next real value uses offset zero. The full arrow/array package, assertion build, compute packages, and IPC package also pass.

[zeroshade]

Verified: `DenseUnionBuilder.AppendNulls`/`AppendEmptyValues` called the child builder once outside the count loop, so a zero count appended no union entry but still advanced the child — I reproduced parent length 0 with child length 1. The `n <= 0` guard is the right fix and brings Dense in line with `SparseUnionBuilder`, which already no-ops on a zero count. LGTM.