Correctly handle null relations on local relation fields

src/index.js

@@ -216,7 +216,13 @@ module.exports = {
 
 			if (remoteTableLayout[foreignKey.column].isPrimaryKey === true) {
 				let field = [ localField, async function (_, { $make, $getProperty }) {
-					return $make(`dlayer-knex.${foreignKey.table}`, { id: await $getProperty(this, name) })
+					let id = await $getProperty(this, name);
+
+					if (id != null) {
+						return $make(`dlayer-knex.${foreignKey.table}`, { id: id });
+					} else {
+						return null;
+					}
 				}];
 
 				selfRelationFields.push(field);