gh-113812: Allow DatagramTransport.sendto to send empty data (#115199)

Also include the UDP packet header sizes (8 bytes per packet)
in the buffer size reported to the flow control subsystem.

Lib/asyncio/transports.py

@@ -181,6 +181,8 @@ class DatagramTransport(BaseTransport):
         to be sent out asynchronously.
         addr is target socket address.
         If addr is None use target address pointed on transport creation.
+        If data is an empty bytes object a zero-length datagram will be
+        sent.
         """
         raise NotImplementedError