fix: fix error in the InMemoryAdapter when using _empty operator on a field with value null

Author: Yogu

src/database/inmemory/js-generator.ts

@@ -430,7 +430,8 @@ register(TransformListQueryNode, (node, context) => {
 });
 
 register(CountQueryNode, (node, context) => {
-    return js`${processNode(node.listNode, context)}.length`;
+    // in arangodb LENGTH(null) is 0, which can be useful sometimes, so let's just mimick the behavior
+    return js`(${processNode(node.listNode, context)}?.length ?? 0)`;
 });
 
 register(AggregationQueryNode, (node, context) => {

src/query-tree/lists.ts

@@ -135,6 +135,8 @@ export class ConcatListsQueryNode extends QueryNode {
 
 /**
  * A node that evaluates to the number of items in a list
+ *
+ * If the listNode evaluates to null, this node evaluates to zero
  */
 export class CountQueryNode extends QueryNode {
     constructor(public readonly listNode: QueryNode) {