Use ids.Empty instead of ids.ID{}

[dhrubabasu]

Why this should be merged

We should be consistent on which one we use

How this works

Aligns on ids.Empty

How this was tested

CI

[meaghanfitzgerald]

Did a global search and it says this variable exists in 36 files, this PR changes 25.
i.e. https://github.com/ava-labs/avalanchego/blob/master/vms/components/avax/utxo_fetching.go

[dhrubabasu]

The remaining 11 hits are []ids.ID{}, not ids.ID{}