Allow nullable fields with "__icontains"

Hi,

I noticed that when some nodes don't have the property used by "__icontains", the `!` that allows nullable property doesn't work properly. It causes a "java.lang.NullPointerException". To avoid that, I added a "HAS" function just before calling the `cypher_predicate_from_condition`.

Note that it is failing such as the upstream but this patch works.

All the best,
Pierre