This commit fixes a cilium-agent panic during datapath reinitialization
when a DirectRouting device is required but not configured.
This can happen when the direct routing device drops, for example during
networkd restart.

```
time=2026-01-14T07:39:46.444386888Z level=info msg="Devices changed" module=agent.datapath.devices-controller devices=[]
time=2026-01-14T07:39:46.444654289Z level=info msg="Fallback node addresses updated" module=agent.datapath.node-address addresses="127.0.0.1 (primary), ::1 (primary)" device=*
time=2026-01-14T07:39:46.44474159Z level=info msg="Node addresses updated" module=agent.datapath.node-address addresses="127.0.0.1 (primary), ::1 (primary)" device=*
time=2026-01-14T07:39:46.444833191Z level=info msg="Node addresses updated" module=agent.datapath.node-address addresses="" device=eth0
panic: runtime error: index out of range [3] with length 0

goroutine 415 [running]:
github.com/cilium/cilium/pkg/byteorder.NetIPv4ToHost32({0x0?, 0xc000e9e5d0?, 0x49e07bb?})
        /go/src/github.com/cilium/cilium/pkg/byteorder/byteorder.go:15 +0x65
github.com/cilium/cilium/pkg/datapath/linux/config.(*HeaderfileWriter).WriteNodeConfig(0xc0004280e0, {0x7ff1517a6ba8, 0xc0023ec400}, 0xc001d00508)
        /go/src/github.com/cilium/cilium/pkg/datapath/linux/config/config.go:150 +0xa4b
github.com/cilium/cilium/pkg/datapath/loader.hashDatapath({0x50fbfb0, 0xc0004280e0}, 0xc001d00508)
        /go/src/github.com/cilium/cilium/pkg/datapath/loader/hash.go:20 +0x9e
github.com/cilium/cilium/pkg/datapath/loader.(*objectCache).UpdateDatapathHash(0xc001d027d0, 0xc001422870?)
        /go/src/github.com/cilium/cilium/pkg/datapath/loader/cache.go:62 +0x4d
github.com/cilium/cilium/pkg/datapath/loader.(*loader).Reinitialize(0xc002573580, {0x50f9c98, 0xc0008474d0}, 0xc001d00508, {{0x49d15b6, 0x4}, {0x0, 0x0}, 0x0, 0x0, ...}, ...)
        /go/src/github.com/cilium/cilium/pkg/datapath/loader/base.go:377 +0x3c8
github.com/cilium/cilium/pkg/datapath/orchestrator.(*orchestrator).reinitialize(0xc001d36288, {0x50f9c98?, 0xc0008474d0?}, {{0x0?, 0x0?}, 0x0?}, 0xc001d00508)
        /go/src/github.com/cilium/cilium/pkg/datapath/orchestrator/orchestrator.go:275 +0x110
github.com/cilium/cilium/pkg/datapath/orchestrator.(*orchestrator).reconciler(0xc001d36288, {0x50f9c98, 0xc0008474d0}, {0x5104260, 0xc002feafc0})
        /go/src/github.com/cilium/cilium/pkg/datapath/orchestrator/orchestrator.go:219 +0x6fd
github.com/cilium/hive/job.(*jobOneShot).start(0xc002082e40, {0x50f9c98, 0xc0008474d0}, 0xc00143dce4?, {0x5104260, 0xc002082de0}, {{{0x0, 0x0, 0x0}}, 0xc001791770, ...})
        /go/src/github.com/cilium/cilium/vendor/github.com/cilium/hive/job/oneshot.go:138 +0x4fd
created by github.com/cilium/hive/job.(*queuedJob).Start.func1 in goroutine 1
        /go/src/github.com/cilium/cilium/vendor/github.com/cilium/hive/job/job.go:126 +0x16f
```

With the change in this commit when a direct routing device is not found
datapath orchestrator will log a warning and wait for device updates in
the reconciliation loop, skipping reinitialization.

Fixes 8fae439 ("datapath: move DirectRoutingDevice validation to orchestrator")

Signed-off-by: Deepesh Pathak <deepeshpathak09@gmail.com>