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

src/System.Device.Gpio/System/Device/I2c/I2cDevice.cs

@@ -70,6 +70,8 @@ public virtual unsafe void WriteByte(byte value)
     /// <param name="buffer">
     /// The buffer that contains the data to be written to the I2C device.
     /// The data should not include the I2C device address.
+    /// An empty buffer generates a transaction on the bus (Start condition, device address and Stop
+    /// condition) without transferring any data byte, which some devices require to be woken up.
     /// </param>
     public abstract void Write(ReadOnlySpan<byte> buffer);
 

src/System.Device.Gpio/System/Device/I2c/UnixI2cBus.cs

@@ -121,6 +121,17 @@ internal unsafe void Write(int deviceAddress, ReadOnlySpan<byte> buffer)
             throw new ArgumentException($"{nameof(buffer)} length is too long.", nameof(buffer));
         }
 
+        if (buffer.Length == 0)
+        {
+            // An empty write still generates a transaction on the bus (Start condition, device
+            // address, Stop condition) without transferring any data byte. Some devices (e.g. the
+            // PN532 NFC reader) rely on this to be woken up. A non-null pointer is required so that
+            // WriteReadCore emits a zero-length write message instead of skipping it.
+            byte placeholder = 0;
+            WriteReadCore((ushort)deviceAddress, &placeholder, null, 0, 0);
+            return;
+        }
+
         fixed (byte* writeBufferPointer = buffer)
         {
             WriteReadCore((ushort)deviceAddress, writeBufferPointer, null, (ushort)buffer.Length, 0);