Fix NPC damage condition

[ShadowDash2000]

There is a chance that the OnDamage() method can be called twice (or more). For example, in the CombatNpc component this may cause a repeat call of Speech.Say() and Die() methods. So, I added an early return condition if NPC's health is below 1.
The Player component is already does this in OnDamage() method:

public void OnDamage( in DamageInfo dmg )
	{
		if ( Health < 1 ) return;
		if ( !PlayerData.IsValid() ) return;
		if ( PlayerData.IsGodMode ) return;
...

I think that Health should ideally be a separate component to avoid repetitive code. Like this one, which I've implemented here.