Handle stream is None case in TCP comm finalizer

[jrbourbeau]

Some Dask users reported running into the following error coming from the TCP comm finalizer method

Exception ignored in: <finalize object at 0x7fc093a9f1a0; dead>
Traceback (most recent call last):

  File "distributed/comm/tcp.py", line 172, in finalize
    if not stream.closed():
AttributeError: 'NoneType' object has no attribute 'closed'

Looking at the TCP comm, there are some code paths where we will set the stream to None:

distributed/distributed/comm/tcp.py

Lines 201 to 205 in 8942856

	except StreamClosedError as e:
	self.stream = None
	self._closed = True
	if not sys.is_finalizing():
	convert_stream_closed_error(self, e)

In particular, when sys.is_finalizing() is True we don't raise an error and could end up with

AttributeError: 'NoneType' object has no attribute 'closed'

when the finalizer for the comm is run during interpreter shutdown.

This PR adds a check to the corresponding finalize function to make sure stream is not None before attempting to call stream.close()