refactor(mutate): simplify work with layers in MutateTime, add test

[sumkincpp]

This PR refactors current code for working with Time Mutation

• It is possible to separate a work with ocf.History and with Layers (non-empty layers only copied).
• a test is added to detect edge case (out of bounds panic, see fix: add bounds checking to addendum layer mutations to prevent panic #1715)

PSS. the role of preparing of non-empty layers in newLayer, err := layerTime(layer, t) is not very clear to me, though it seems that we are taking only time from known next found layer. if they are initially collected without empty layers, then it seems the correct way to do

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.