Python Issue with Optional UUIDs

[Daniel Young]

Hi,
I believe there is an issue with the python library supporting a schema with 
optional UUIDs.
The field should look like:

"fields": [
    {
      "name": "userId",
      "type": [
        "null",
        {
          "type": "string",
          "logicalType": "uuid"
        }
      ]
    }

]

but currently that does not validate with a userId: None and gives a TypeError 
exception.

I believe the fix is to catch TypeError here: 
https://github.com/apache/avro/blob/master/lang/py/avro/schema.py#L1046 in the 
same way that ValueError is caught. Anything that doesn't match the appropriate 
UUID type is guaranteed to not be a UUID.
I didn't find this in the issue tracker.
Thanks,
Daniel Young