Handle empty devices in OCI spec

[zvonkok]

Handle empty devices in OCI spec, if OCI spec has an empty linux.resource.devices entry aka None the CDI injection will fail since we are checking for Some(devices).

If devices is None create an vector with the new device and set it in the resources section.

With

cdiVersion: 0.5.0
containerEdits:
  deviceNodes:
  - path: /dev/nvidia0
  - path: /dev/nvidiactl

we get

    "linux": {
      "resources": {
        "devices": [
          {
            "allow": true,
            "type": "c",
            "major": 195,
            "minor": 0,
            "access": "rwm"
          },
          {
            "allow": true,
            "type": "c",
            "major": 195,
            "minor": 0,
            "access": "rwm"
          },
          {
            "allow": true,
            "type": "c",
            "major": 242,
            "minor": 0,
            "access": "rwm"
          }
        ]
      },

Without the patch we got

   "linux": {
      "resources": {},

[zvonkok]

@Apokleos PTAL