When user call the `Call()` method, they don't always want to sent
some args or get the return value, so they use `nil` when call `Call()`
method and this will casue an error. It's better to not trying to
encode or decode if it's nil.
Signed-off-by: Lei Jitang <leijitang@huawei.com>
Upstream-commit: a2d348968fe754df41d0a1b56c7ba29ebbaa6849
Component: engine