config-linux: Make linux.resources.devices explicitly optional

And mark it omitempty to avoid:

  $ ocitools generate --template <(echo '{"linux": {"resources": {}}}') | jq .linux
  {
    "resources": {
      "devices": null
    }
  }

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config-linux.md

@@ -198,7 +198,7 @@ However, a runtime MAY attach the container process to additional cgroup control
 
 #### Device whitelist
 
-`devices` is an array of entries to control the [device whitelist][cgroup-v1-devices].
+**`devices`** (array, optional) configures the [device whitelist][cgroup-v1-devices].
 The runtime MUST apply entries in the listed order.
 
 The following parameters can be specified:

specs-go/config.go

@@ -301,8 +301,8 @@ type Network struct {
 
 // Resources has container runtime resource constraints
 type Resources struct {
-	// Devices are a list of device rules for the whitelist controller
-	Devices []DeviceCgroup `json:"devices"`
+	// Devices configures the device whitelist.
+	Devices []DeviceCgroup `json:"devices,omitempty"`
 	// DisableOOMKiller disables the OOM killer for out of memory conditions
 	DisableOOMKiller *bool `json:"disableOOMKiller,omitempty"`
 	// Specify an oom_score_adj for the container.