fix: Write zero-filled values buffer for fully-null Bool columns in IPC

[AlonSpivack]

Summary

When a Bool column is fully null (nullCount >= length), assembleBoolVector in
VectorAssembler previously returned early without writing a values buffer,
producing an IPC stream with 0 bytes for buffer #1 .

This violates the Arrow IPC specification, which requires a data buffer of
ceil(length / 8) bytes for Bool arrays regardless of null count. Other
implementations (PyArrow, arrow-rs) reject these streams with:

Buffer #1 too small in array of type Bool. Expected at least 1 byte(s), got 0

Fix

Write a zero-filled Uint8Array of the correct byte length ((data.length + 7) >> 3)
when all values are null, instead of returning early with no buffer.

Tests

Added 4 round-trip tests for fully-null Bool columns through tableToIPC / tableFromIPC:

• Single-row fully-null Bool
• 2-row fully-null Bool (file format)
• 10-row fully-null Bool (crosses byte boundary)
• Mixed table with normal Int32 + fully-null Bool columns

Closes #68

cc @trxcllnt @domoritz