[pLz5YbCW] add checks for node labels and relationship types for apoc.merge procedures

[nadja-muller]

apoc.merge.node:

• if labelNames null is newly considered as an empty label
• if labelNames contains null values, it is considered as incorrect input
• if labelNames contains among others an empty string, it is considered as incorrect input

apoc.merge.relationship:

• if relationship type is null, it is considered as incorrect input
• if relationship type is empty, it is considered as incorrect input

cherry-pick: neo4j/apoc#506