Don't assume plotter params exist

There seems to be an edge case when running the dev setup locally where there aren't any default plotter parameters available for the plotter. This crashes the search functionality. To reproduce:
- Go to an experiment
- Click on the blue magnifying glass to search for relevant experiments
- Django should crash in `search_tags.py` giving `'NoneType' object has no attribute 'fullname'`

See merge request !215