Fire BlockPistonRetractEvent for all empty pistons

There is an explicit check in the handling code for empty pistons that
prevents sticky pistons from firing the event. However when we look back
at the history we see that this check was originally added so that ONLY
sticky pistons would fire the retract event. I'm not sure why.
1092acbddf

Over the course of several updates, the meaning of that field appears to
have changed from "is NOT sticky" to "is sticky". So now its having the
opposite effect. Only normal pistons fire the retraction event. And like
all things in CB, it's just been carried around since.

If we are to believe the history, the correct fix for this issue is to
flip it so it only fires for sticky pistons, but that puts us in a
bind. It's already firing for non-sticky pistons, changing it now would
likely result in breakage. Furthermore, there is little documentation as
to WHY that was ever intended to be the case.

Instead we opt to remove the check entirely so that the event fires for
all piston types.

Fixes GH-1829

Spigot-Server-Patches/0415-Fire-BlockPistonRetractEvent-for-all-empty-pistons.patch

@@ -0,0 +1,50 @@
+From 823c5e876e5dc00680950023057a21894c12011c Mon Sep 17 00:00:00 2001
+From: Zach Brown <zach@zachbr.io>
+Date: Thu, 31 Jan 2019 16:33:36 -0500
+Subject: [PATCH] Fire BlockPistonRetractEvent for all empty pistons
+
+There is an explicit check in the handling code for empty pistons that
+prevents sticky pistons from firing the event. However when we look back
+at the history we see that this check was originally added so that ONLY
+sticky pistons would fire the retract event. I'm not sure why.
+https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/commits/1092acbddf07edfa4100bc6824504ac75088e913
+
+Over the course of several updates, the meaning of that field appears to
+have changed from "is NOT sticky" to "is sticky". So now its having the
+opposite effect. Only normal pistons fire the retraction event. And like
+all things in CB, it's just been carried around since.
+
+If we are to believe the history, the correct fix for this issue is to
+flip it so it only fires for sticky pistons, but that puts us in a
+bind. It's already firing for non-sticky pistons, changing it now would
+likely result in breakage. Furthermore, there is little documentation as
+to WHY that was ever intended to be the case.
+
+Instead we opt to remove the check entirely so that the event fires for
+all piston types.
+
+diff --git a/src/main/java/net/minecraft/server/BlockPiston.java b/src/main/java/net/minecraft/server/BlockPiston.java
+index dbe0ff33a..0109484e0 100644
+--- a/src/main/java/net/minecraft/server/BlockPiston.java
++++ b/src/main/java/net/minecraft/server/BlockPiston.java
+@@ -118,7 +118,7 @@ public class BlockPiston extends BlockDirectional {
+             }
+ 
+             // CraftBukkit start
+-            if (!this.sticky) {
++            //if (!this.sticky) { // Paper - Prevents empty sticky pistons from firing retract - history behind is odd
+                 org.bukkit.block.Block block = world.getWorld().getBlockAt(blockposition.getX(), blockposition.getY(), blockposition.getZ());
+                 BlockPistonRetractEvent event = new BlockPistonRetractEvent(block, ImmutableList.<org.bukkit.block.Block>of(), CraftBlock.notchToBlockFace(enumdirection));
+                 world.getServer().getPluginManager().callEvent(event);
+@@ -126,7 +126,7 @@ public class BlockPiston extends BlockDirectional {
+                 if (event.isCancelled()) {
+                     return;
+                 }
+-            }
++            //} // Paper
+             // PAIL: checkME - what happened to setTypeAndData?
+             // CraftBukkit end
+             world.playBlockAction(blockposition, this, b0, enumdirection.a());
+-- 
+2.20.1
+