Some result sets include `None` values

[joshuathayer]

eg

payload={'target-column': 'Opioid_Deaths', 'context-columns': ['"Total Property Crime Arrests Rate (per 100000 Population)"']}
headers={'content-type': 'application/json'}
response = requests.post('http://bayesrest:5000/find-anomalies', json=payload, headers=headers)

determine where the Nones are coming from. If we're getting suspect data from bayeslite, file an issue there.