return NULL on EntityManager::load()/loadMultiple() with invalid ID / non existing one

Follow-up to #2403271: return FALSE on EntityManager::loadEntityByUuid with invalid UUID / non existing one

Problem/Motivation

EntityManager::load()/loadMultiple() breaks if you pass in a wrong / non existing UUID.

Proposed resolution

Let's return FALSE, in case we have a wrong one.

Remaining tasks

User interface changes

API changes