null bean evaluation

[leontyevdv]

Hello Romain! Thanks for this useful library! I use it for filtering rows in Swing table and have a trouble. Some getters return null value. E.g Double getSomeValue() method may return null value. Method fr.inria.jfilter.operators.ComparableFilter#eval rises NPE while executes. Can you fix it? Thanks!

Another one question: Does your library support null-filtering? I'd like to filter beans which has NULL in particular fields.

[rouvoy]

Thanks for reporting an issue. Would that be possible for you to share a unit test exposing this incorrect behaviour? Thanks.

[leontyevdv]

Sent you PR with test and bug fix