fix: reduceRef to return undefined node when node not given or not found

Author: sagold

src/keywords/$ref.ts

@@ -74,7 +74,15 @@ export function parseRef(node: SchemaNode) {
 }
 
 export function reduceRef({ node, data, key, pointer, path }: JsonSchemaReducerParams) {
+    if (node == null) {
+        return;
+    }
+
     const resolvedNode = node.resolveRef({ pointer, path });
+    if (resolvedNode == null) {
+        return;
+    }
+
     if (resolvedNode.schemaId === node.schemaId) {
         return resolvedNode;
     }