fix each block validation on strings (#4450)

src/runtime/internal/dev.ts

@@ -80,7 +80,7 @@ export function set_data_dev(text, data) {
 }
 
 export function validate_each_argument(arg) {
-	if (!arg || !('length' in arg)) {
+	if (typeof arg !== 'string' && !(arg && typeof arg === 'object' && 'length' in arg)) {
 		let msg = '{#each} only iterates over array-like objects.';
 		if (typeof Symbol === 'function' && arg && Symbol.iterator in arg) {
 			msg += ' You can use a spread to convert this iterable into an array.';