null tag causes TypeError: 'NoneType' object is not iterable

[Bjwebb]

Can be reproduce for OCDS Cove with:

{"releases":[{"tag":null}]}

From Sentry:
https://app.getsentry.com/open-data-services/cove/issues/122978992/

TypeError: 'NoneType' object is not iterable
(31 additional frame(s) were not displayed)
...
  File "cove/views.py", line 617, in wrapper
    return func(request, *args, **kwargs)
  File "cove/views.py", line 850, in explore
    return render(request, view, context)

[Bjwebb]

Fix available for testing at http://release-201603.dev.cove.opendataservices.coop/

Hoping to roll this out tomorrow