Allow explosions under 0 height (Fixes #5974)

Upstream forgot to update the check here which prevents explosions
occuring under 0 height, however, vanilla already checks that we
are in bounds of the world above this, so this call is entirely
redundant.

patches/server/0721-Allow-explosions-under-0-height.patch

@@ -0,0 +1,23 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Thu, 24 Jun 2021 01:42:35 +0100
+Subject: [PATCH] Allow explosions under 0 height
+
+Upstream forgot to update the check here which prevents explosions
+occuring under 0 height, however, vanilla already checks that we
+are in bounds of the world above this, so this call is entirely
+redundant.
+
+diff --git a/src/main/java/net/minecraft/world/level/Explosion.java b/src/main/java/net/minecraft/world/level/Explosion.java
+index d4785160c1477ed13343e30c913b08175011d048..fb50ee3daba9f4fe8cc173c209521679a59f23d7 100644
+--- a/src/main/java/net/minecraft/world/level/Explosion.java
++++ b/src/main/java/net/minecraft/world/level/Explosion.java
+@@ -190,7 +190,7 @@ public class Explosion {
+                                 f -= ((Float) optional.get() + 0.3F) * 0.3F;
+                             }
+ 
+-                            if (f > 0.0F && this.damageCalculator.shouldBlockExplode(this, this.level, blockposition, iblockdata, f) && blockposition.getY() < 256 && blockposition.getY() >= 0) { // CraftBukkit - don't wrap explosions
++                            if (f > 0.0F && this.damageCalculator.shouldBlockExplode(this, this.level, blockposition, iblockdata, f)/* && blockposition.getY() < 256 && blockposition.getY() >= 0*/) { // CraftBukkit - don't wrap explosions // Paper - permit < 0
+                                 set.add(blockposition);
+                                 // Paper start - prevent headless pistons from forming
+                                 if (!com.destroystokyo.paper.PaperConfig.allowHeadlessPistons && iblockdata.getBlock() == Blocks.MOVING_PISTON) {