Package info fails with releases without tag

[javierggt]

I didn't even know this could happen.

this acis_thermal_check release seems to have no associated tag. The package info query then fails in _get_tag_target because the tag is None:

TypeError                                 Traceback (most recent call last)
<ipython-input-25-52b6a5bef947> in <module>
----> 1 packages._get_tag_target(r['tag'])

~/SAO/miniconda3-shiny/envs/ska3-shiny/lib/python3.8/site-packages/skare3_tools/packages.py in _get_tag_target(tag)
    193 
    194 def _get_tag_target(tag):
--> 195     if 'target' in tag:
    196         return _get_tag_target(tag['target'])
    197     else:

TypeError: argument of type 'NoneType' is not iterable