extmod: Make ujson.loads raise exception if given empty string.

Addresses issue #1097.

extmod/modujson.c

@@ -239,7 +239,8 @@ STATIC mp_obj_t mod_ujson_loads(mp_obj_t obj) {
         // unexpected chars
         goto fail;
     }
-    if (stack.len != 0) {
+    if (stack_top == MP_OBJ_NULL || stack.len != 0) {
+        // not exactly 1 object
         goto fail;
     }
     vstr_clear(&vstr);

tests/extmod/ujson_loads.py

@@ -33,3 +33,9 @@ my_print(json.loads('{"a":[], "b":[1], "c":{"3":4}}'))
 
 # whitespace handling
 my_print(json.loads('{\n\t"a":[]\r\n, "b":[1], "c":{"3":4}     \n\r\t\r\r\r\n}'))
+
+# loading nothing should raise exception
+try:
+    json.loads('')
+except ValueError:
+    print('ValueError')