Called before the first call to OnUpdate and when a system resumes updating after being stopped or disabled.
If the EntityQuery objects defined for a system do not match any existing entities then the system skips updates until a successful match is found. Likewise, if you set ComponentSystemBase.Enabled to false, then the system stops running. In both cases, ComponentSystemBase.OnStopRunning is called when a running system stops updating; OnStartRunning is called when it starts updating again.