Fix AttributeError when client.get_default_node() returns None

[bacchuswng]

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Description of change

The default node is sometimes None. It's not obvious from the code how this happens (maybe an Exception that isn't being handled correctly, or a connection-related object is being closed while pipeline execution is still in progress). Regardless, we've observed AttributeError: 'NoneType' object has no attribute 'name' in practice.

Adding this defensive check ensures that we don't run into this issue.

[zs-neo]

Can you provide more information? How do you use redis-py? We are also trying related features and want to make sure we don’t encounter the same error.