Allow zero revive health when it matches maxHealth

Apparently a zero max health attribute is perfectly fine in vanilla and
our own revive handling code appears to handle the case fine, even when
EntityDeathEvent is cancelled. So we should allow it to avoid issues
when these mobs are killed.

Spigot-API-Patches/0149-Improve-death-events.patch

@@ -1,4 +1,4 @@
-From c3e3a14ed619bbe7b1eb5af0a11c333dd495652b Mon Sep 17 00:00:00 2001
+From aef4c17d56b4333e03d4a071b0f3fd61a7c9bab8 Mon Sep 17 00:00:00 2001
 From: Phoenix616 <mail@moep.tv>
 Date: Tue, 21 Aug 2018 01:32:28 +0100
 Subject: [PATCH] Improve death events
@@ -15,7 +15,7 @@ items and experience which is otherwise only properly possible by using
 internal code.
 
 diff --git a/src/main/java/org/bukkit/event/entity/EntityDeathEvent.java b/src/main/java/org/bukkit/event/entity/EntityDeathEvent.java
-index ab9e81fd2..a7b8f869c 100644
+index ab9e81fd..fef134c6 100644
 --- a/src/main/java/org/bukkit/event/entity/EntityDeathEvent.java
 +++ b/src/main/java/org/bukkit/event/entity/EntityDeathEvent.java
 @@ -8,10 +8,19 @@ import org.bukkit.inventory.ItemStack;
@@ -74,7 +74,7 @@ index ab9e81fd2..a7b8f869c 100644
 +     */
 +    public void setReviveHealth(double reviveHealth) throws IllegalArgumentException {
 +        double maxHealth = ((LivingEntity) entity).getAttribute(org.bukkit.attribute.Attribute.GENERIC_MAX_HEALTH).getValue();
-+        if ((reviveHealth <= 0) || (reviveHealth > maxHealth)) {
++        if ((maxHealth != 0 && reviveHealth <= 0) || (reviveHealth > maxHealth)) {
 +            throw new IllegalArgumentException("Health must be between 0 (exclusive) and " + maxHealth + " (inclusive), but was " + reviveHealth);
 +        }
 +        this.reviveHealth = reviveHealth;