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

It used to have this, but the omitempty was dropped in 652323c
(improve seccomp format to be more expressive, 2017-01-13, opencontainers#657).
However, the docs that landed in 3ca5c6c (config-linux.md: fix
seccomp, 2017-03-02, opencontainers#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 <wking@tremily.us>

specs-go/config.go

@@ -551,7 +551,7 @@ type LinuxSeccompArg struct {
 type LinuxSyscall struct {
 	Names  []string           `json:"names"`
 	Action LinuxSeccompAction `json:"action"`
-	Args   []LinuxSeccompArg  `json:"args"`
+	Args   []LinuxSeccompArg  `json:"args,omitempty"`
 }
 
 // LinuxIntelRdt has container runtime resource constraints