GH-343: Fix BaseVariableWidthVector and BaseLargeVariableWidthVector offset buffer serialization

[Yicong-Huang]

What's Changed

Fix BaseVariableWidthVector/BaseLargeVariableWidthVector IPC serialization when valueCount is 0.

Problem

When valueCount == 0, setReaderAndWriterIndex() was setting offsetBuffer.writerIndex(0), which means readableBytes() == 0. IPC serializer uses readableBytes() to determine buffer size, so 0 bytes were written to the IPC stream. This crashes IPC readers in other libraries because Arrow spec requires offset buffer to have at least one entry [0].

This is a follow-up to #967 which fixed the same issue in ListVector/LargeListVector.

Fix

Simplify setReaderAndWriterIndex() to always use (valueCount + 1) * OFFSET_WIDTH for offset buffer's writerIndex. When valueCount == 0, this correctly sets writerIndex to OFFSET_WIDTH, ensuring offset[0] is included in serialization.

Testing

Added tests for empty VarCharVector and LargeVarCharVector verifying offset buffer has correct readableBytes().

related to #343

[Yicong-Huang]

cc @viirya @jbonofre