Support for `undefined` on `.where`

[angrykoala]

Example use case:

const filterPredicates: Cypher.Predicate | undefined = this.getPredicates(context);

const clause = new Cypher.Match(nodeVar).where(filterPredicates).return(projectionMap);

In this case, the resulting cypher should omit the WHERE:

MATCH(m)
RETURN {}