[SPIGOT-424, MC-73474] Add fix for powered rail duplication.

No redstone related side effects that we know of and hopefully there are none. Please report any to the tracker.

nms-patches/BlockMinecartTrackAbstract.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/BlockMinecartTrackAbstract.java
++++ b/net/minecraft/server/BlockMinecartTrackAbstract.java
+@@ -87,7 +87,7 @@
+                 flag = true;
+             }
+ 
+-            if (flag) {
++            if (flag && !world.isEmpty(blockposition)) { // CraftBukkit - SPIGOT-424, MC-73474
+                 this.b(world, blockposition, iblockdata, 0);
+                 world.setAir(blockposition);
+             } else {