drivenets: create empty ConfigMap when config mount is defaulted

[msupinodn]

Summary

cdnosDefaults populates pb.Config.ConfigPath (/config_load) and pb.Config.ConfigFile (default) on every node, but CreateConfig short-circuits and creates no ConfigMap when no startup data (Config_File/Config_Data) is supplied.

The cdnos-controller still mounts the <node>-config ConfigMap into the pod whenever ConfigPath + ConfigFile are populated on the CR. With the ConfigMap missing, every pod sits forever in PodInitializing with:

MountVolume.SetUp failed for volume "config":
  configmap "<node>-config" not found

This shows up immediately on any topology whose nodes don't ship a startup configuration — for example lightweight scale tests using an Alpine image where data:/file: is omitted from the proto.

Fix

When ConfigPath + ConfigFile are populated but no data was provided, create an empty ConfigMap (single empty key) instead of returning nil. Users who explicitly want to opt out of the config mount can still clear ConfigPath/ConfigFile in the proto, and the existing "no mount" path is preserved.

Test plan

• New unit test TestCreateConfig_NoDataButPathDefaulted — defaulted CR creates the ConfigMap.
• New unit test TestCreateConfig_NoDataAndNoPath — explicit opt-out still skips ConfigMap creation.
• go vet ./topo/node/drivenets/ clean.
• Reproduced end-to-end against a 200-node Alpine kne topology — without this fix every pod hangs in PodInitializing; with it, pods come up cleanly.

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