Prevent NPE on ThrownPotion#splash

[SoSeDiK]

ThrownPotion#splash calls internal splash with null hit result:

Paper/paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftThrownPotion.java

Lines 48 to 51 in 3f5728e

	@Override
	public void splash() {
	this.getHandle().splash(null);
	}

Which leads to NPE in case of thrown non-water splash potions. Other parts of code expect and handle nullability, but this one was missed.