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/proactor_events.py

@@ -487,9 +487,6 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
             raise TypeError('data argument must be bytes-like object (%r)',
                             type(data))
 
-        if not data:
-            return
-
         if self._address is not None and addr not in (None, self._address):
             raise ValueError(
                 f'Invalid address: must be None or {self._address}')
@@ -502,7 +499,7 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
 
         # Ensure that what we buffer is immutable.
         self._buffer.append((bytes(data), addr))
-        self._buffer_size += len(data)
+        self._buffer_size += len(data) + 8  # include header bytes
 
         if self._write_fut is None:
             # No current write operations are active, kick one off