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

Lib/asyncio/selector_events.py

@@ -1241,8 +1241,6 @@ class _SelectorDatagramTransport(_SelectorTransport, transports.DatagramTranspor
         if not isinstance(data, (bytes, bytearray, memoryview)):
             raise TypeError(f'data argument must be a bytes-like object, '
                             f'not {type(data).__name__!r}')
-        if not data:
-            return
 
         if self._address:
             if addr not in (None, self._address):
@@ -1278,7 +1276,7 @@ class _SelectorDatagramTransport(_SelectorTransport, transports.DatagramTranspor
 
         # 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
         self._maybe_pause_protocol()
 
     def _sendto_ready(self):

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