Tandemvault image importer - possibility to divide by zero when calculating image's mean confidence score

[cjg89]

https://github.com/UCF/Search-Service-Django/blob/master/images/management/commands/import-tandemvault-images.py#L284-L287

It's possible for a ZeroDivisionError error to occur here if data['labels'] is empty. Add some logic here to avoid dividing by zero if data['labels'] is empty by that point.