Don't count empty steps as proper steps

Problem/Motivation

There is 2 ways of creating an instance:

• bare bone, with EntityInterface::create() or with EntityStorageInterface::create(). We use this only in kernel tests as far as I know
• so through a DisplayBuildable plugin

Only the creation through a DisplayBuildable initialize a present state, the bare bone is initializing an empty array.

But this array is move around when we manipulate history and Instance::getCountPast() and Instance::getCountFuture() count this empty array.

Proposed resolution

Remove empty arrays from Instance::getCountPast() and Instance::getCountFuture(), and also prevent them to be moved around.

Fix false positives from InstanceHistoryTest