Error when image size data has no value

While investigating a separate issue, I found the following error message being repeatedly reported in the journal/log:

Traceback (most recent call last):
  File "/snap/cloudstats/x1/bin/cloudstats-exporter", line 33, in <module>
    sys.exit(load_entry_point('cloudstats==0.1', 'console_scripts', 'cloudstats-exporter')())
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/exporter.py", line 54, in main
    daemon.run()
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/exporter.py", line 47, in run
    self.trigger()
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/exporter.py", line 38, in trigger
    self.openstack.get_all_stats()
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/opensdk.py", line 230, in get_all_stats
    self._get_image_stats()
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/opensdk.py", line 375, in _get_image_stats
    value=image["size"],
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/cloudstats/opensdk.py", line 286, in _create_or_update_gauge
    self.gauge_dict[gauge_name].labels(**labels).set(value)
  File "/snap/cloudstats/x1/lib/python3.6/site-packages/prometheus_client/metrics.py", line 359, in set
    self._value.set(float(value))
TypeError: float() argument must be a string or a number, not 'NoneType'