added support for passing null parameters to Gremlin queries

I needed to retrieve all of the predicates on inbound links to a node using a query like the following:

    g.V("12").In().Tag(null,"link-type").All()

However, cayley-clj was filtering out the null parameter and was instead sending the following query:

    g.V("12").In().Tag("link-type").All()

This pull request includes code to support passing null parameters using nil:

    (cayley/query [[:g][:V [node-id]][:In [nil, "link-type"]][:All []]] query-url)