Fix case where field list contains empty value

[horde3d]

I'm actually not sure what the root cause is, that in my installation the graph query contains an empty field (might depend on a custom facet based on a regex I've added) but there is an issue in case the query for the visual graph explorer contains a field list with an empty value.

To fix this I would propose to add a one liner to

open-semantic-visual-graph-explorer/src/visual_graph_explorer/views.py

Line 41 in 7fcd063

fields = [x for x in fields if x.strip()]

this would remove all empty fields from the query, so it does not cause an error anymore.