"Value is none." logged unnecessarily

[ajdavis]

AtlasMeasurementValue has code to handle null values from the Atlas API, but it nevertheless will try to execute float(value) even when value is None. If there were an else here we would see fewer "Value is none." messages:

https://github.com/mgmonteleone/python-atlasapi/blob/master/atlasapi/measurements.py#L251