fix(Transformer): handle undefined values better

The transformer was incorrectly handling undefined values. While Neo4j does not have the concept
of undefined values (instead only using null), the Neo4j driver does sometimes represent an object
with a property that is undefined such as the [Point](https://github.com/neo4j/neo4j-javascript-driver/blob/4.0/types/spatial-types.d.ts#L27)
object.

Author: bboure

src/transformer.ts

@@ -64,7 +64,7 @@ export class Transformer {
 
   private isPlainValue(value: any): value is PlainValue {
     const type = typeof value;
-    return value === null || type === 'string' || type === 'boolean' || type === 'number';
+    return value == null || type === 'string' || type === 'boolean' || type === 'number';
   }
 
   private isNode(node: any): node is NeoNode {