twn_util: dont add empty list

src/twn_util.c

@@ -233,6 +233,9 @@ String file_read(char const *file, char const *operation) {
             return (String){NULL, 0};
         }
 
+        if (list.list == NULL)
+            return (String){NULL, 0};
+
         String s = {(char *)list.list, (float)list.length - 1};
         arrpush(read_files, s);