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>
Allows people to create out-of-process graphdrivers that can be used
with Docker.
Extensions must be started before Docker otherwise Docker will fail to
start.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
