collectd send NaN value and that's not supported by Gnocchi

[sileht]

[2018-02-08 14:41:18] Traceback (most recent call last):
[2018-02-08 14:41:18] File "/usr/lib/python2.7/site-packages/collectd_gnocchi/__init__.py", line 34, in inner_func
    return func(*args, **kwargs)
[2018-02-08 14:41:18] File "/usr/lib/python2.7/site-packages/collectd_gnocchi/__init__.py", line 171, in flush
    measures, create_metrics=True)
[2018-02-08 14:41:18] File "/usr/lib/python2.7/site-packages/gnocchiclient/v1/metric.py", line 235, in batch_resources_metrics_measures
    data=ujson.dumps(measures),
[2018-02-08 14:41:18] OverflowError: Invalid Nan value when encoding double