Added guard to prevent null exception on missing properties

[omarzouk]

as per this issue: https://github.com/neo4j-contrib/neo4j-elasticsearch/issues/31 when adding nodes that have missing properties, the transaction fails because of null access exception due to lack of checks. I added a guard, now it works.

[omarzouk]

this actually triggered an issue similar to this: https://github.com/neo4j/neo4j/issues/6685 , not sure how it happened, but it did

[jexp]

Thanks a lot.