Strange thumbnail: false condition in the filter-mapper.js

[marcoleorato]

We had a small issue with the panosc search api at MAX IV, this might be a change happened with the new backend.

When the search api is preparing the filters to query SciCat published data there is a condition for the field thumbnail to be "false" this has been an issue for a few reason, firstly, being that the thumbnail filter is a string meaning the field would be either an empty string or a base64 representation of the thumbnail meaning that no results are ever returned...

This is related to this PR #217 although I feel the idea was to remove the thumbnail from the result not to completely remove the dataset if it has a thumbnail