Fix indexer behavior when no kvblock-keys are generated

[vMaroon]

Summary

kvcache.Indexer to return empty-scores if kvblock.TokenProcessor does not generate keys.

[yankay]

If the score returns nil, then llm-d-inference-scheduler might also need some adjustments. Add some checks for nil.

https://github.com/llm-d/llm-d-inference-scheduler/blob/main/pkg/plugins/scorer/utils.go#L16

[vMaroon]

@yankay agreed that it can be improved there as well - although the current behavior will just be a few useless calls.

[vMaroon]

I have tested that this works with the inference-scheduler as-is. Moving in for v0.3.1 due to urgency.

/lgtm
/approve