fix(core): Fix typings for EntityHydrator with nullable relations

packages/core/src/common/types/entity-relation-paths.ts

@@ -27,7 +27,7 @@ export type EntityRelationPaths<T extends VendureEntity> =
     | TripleDotPath;
 
 export type EntityRelationKeys<T extends VendureEntity> = {
-    [K in Extract<keyof T, string>]: Required<T>[K] extends VendureEntity
+    [K in Extract<keyof T, string>]: Required<T>[K] extends VendureEntity | null
         ? K
         : Required<T>[K] extends VendureEntity[]
         ? K