specs-go/config: Add omitempty to LinuxSyscall.Args
specs-go/config: Add omitempty to LinuxSyscall.Args

It used to have this, but the omitempty was dropped in 652323cd
(improve seccomp format to be more expressive, 2017-01-13, #657).
However, the docs that landed in 3ca5c6c5 (config-linux.md: fix
seccomp, 2017-03-02, #706) list the property as optional, and if it is
optional, we can leave it unset instead of serializing an empty array.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

laijs/specs,hqhq/specs,opencontainers/runtime-spec,opencontainers/specs,wking/opencontainer-runtime-spec,hqhq/runtime-spec,wking/opencontainer-specs