Support zero-length I2C writes to emit a Start/Stop bus transaction

[Copilot]

Some devices (e.g. the PN532 NFC reader) are woken up by an I2C transaction that carries no data — a bare START + device address + STOP. An empty Write previously produced no bus activity (or Error 22 on the file-transfer path) because an empty span yields a null pointer under fixed, and WriteReadCore skips write messages when the write pointer is null.

Changes

• UnixI2cBus.Write — On an empty buffer, pass a non-null placeholder pointer with length 0 so WriteReadCore emits a zero-length write message (START + address + STOP). Non-empty writes are unchanged. The file-transfer fallback bus is covered automatically, since its existing writeBuffer != null branch now runs with length 0.
• I2cDevice.Write — Documented that an empty buffer generates a bus transaction without transferring data bytes.

Usage

// Wake up a device that pings on an empty transaction (e.g. PN532)
device.Write(ReadOnlySpan<byte>.Empty);

Notes

No automated test is included: this path requires real /dev/i2c hardware, and the internal UnixI2cBus has no existing mockable test seam. Behavior was verified with a standalone harness — an empty write emits a len=0 message; normal writes are unaffected.

[Copilot]

Pull request overview

Enables “empty I2C write” semantics on Unix so callers can intentionally emit a START + address + STOP transaction (no data bytes), which some devices use as a wake-up signal.

Changes:

• Update UnixI2cBus.Write to treat buffer.Length == 0 as a real bus transaction by passing a non-null pointer with len = 0 into WriteReadCore.
• Update I2cDevice.Write XML documentation to mention empty-buffer behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/System.Device.Gpio/System/Device/I2c/UnixI2cBus.cs	Adds explicit empty-write handling so WriteReadCore emits a zero-length write message instead of skipping the write.
src/System.Device.Gpio/System/Device/I2c/I2cDevice.cs	Updates Write(ReadOnlySpan<byte>) documentation to describe empty-buffer semantics.