Empty value in multi-value filter doesn't match

[Nicklas Nordborg]

This happens for all list pages for all items which have a property that can be null. To test this create, for example, three bio-sources with description, 'a', 'b' and (empty).

Try the following filters:

• '=': matches the biosource with the empty description
• '<>': matches the biosource with non-empty descriptions

'a

b': matches onle the biosources with 'a' or 'b', but should also match the one with an empty description.