add omitempty to 'Device' and 'Namespace'

Signed-off-by: liangchenye <liangchenye@huawei.com>

config.md

@@ -404,12 +404,7 @@ Here is a full example `config.json` for reference.
             {
                 "type": "mount"
             }
-        ],
-        "devices": null,
-        "seccomp": {
-            "defaultAction": "",
-            "architectures": null
-        }
+        ]
     }
 }
 ```

specs-go/config.go

@@ -128,9 +128,9 @@ type Linux struct {
 	// If resources are specified, the cgroups at CgroupsPath will be updated based on resources.
 	CgroupsPath *string `json:"cgroupsPath,omitempty"`
 	// Namespaces contains the namespaces that are created and/or joined by the container
-	Namespaces []Namespace `json:"namespaces"`
+	Namespaces []Namespace `json:"namespaces,omitempty"`
 	// Devices are a list of device nodes that are created for the container
-	Devices []Device `json:"devices"`
+	Devices []Device `json:"devices,omitempty"`
 	// Seccomp specifies the seccomp security settings for the container.
 	Seccomp *Seccomp `json:"seccomp,omitempty"`
 	// RootfsPropagation is the rootfs mount propagation mode for the container.