fix: don't probe empty CD drives

[smira]

Fixes #81

[smira]

Testing with Talos, with boot CD present:

metadata:
    namespace: runtime
    type: DiscoveredVolumes.block.talos.dev
    id: sr0
    version: 1
    owner: block.DiscoveryController
    phase: running
    created: 2024-04-05T15:39:31Z
    updated: 2024-04-05T15:39:31Z
spec:
    type: disk
    devicePath: /sys/devices/pci0000:00/0000:00:1f.2/ata3/host2/target2:0:0/2:0:0:0/block/sr0
    size: 93255680
    sectorSize: 2048
    ioSize: 2048
    name: iso9660
    label: ISOIMAGE
    blockSize: 2048
    filesystemBlockSize: 2048
    probedSize: 93255680

When CD is not present, sr0 doesn't appear.

[smira]

/m