jsonObject.optString
getString(String name)与optString(String name)，功能一样，只是当无返回值时，getString(String name)抛出错误，optString(String name)返回空值
Returns the value mapped by name if it exists, coercing it if necessary, or throws if no such mapping exists.
Returns the value mapped by name if it exists, coercing it if necessary, or the empty string if no such mapping exists.
