…offset buffer serialization (#989)

## 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

Modify `setReaderAndWriterIndex()` to always use `(valueCount + 1) *
OFFSET_WIDTH` for the offset buffer's `writerIndex`, moved outside the
if/else branch. When the offset buffer capacity is insufficient (e.g.,
empty buffer from constructor or loaded via `loadFieldBuffers()`), it
reallocates a properly sized buffer on demand.


### Testing

Added tests for empty `VarCharVector` and `LargeVarCharVector` verifying
offset buffer has correct `readableBytes()` after `setValueCount(0)`.


Closes #343

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Co-authored-by: Yicong Huang <[email protected]>