Fix missed diff causing lava to move too quickly

Missed diff from old patch file was causing lava to always move at the
faster 'nether' speed, ignoring the slower overworld speed entirely.
This is why we use obfuscation helpers now.

Fixes GH-521

Spigot-Server-Patches/Configurable-lava-flow-speed.patch

@@ -29,9 +29,21 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
       */
      public int getFlowSpeed(World world, BlockPosition blockposition) {
 +        if (this.material == Material.LAVA) {
-+            return world.worldProvider.m() ? world.paperConfig.lavaFlowSpeedNether : world.paperConfig.lavaFlowSpeedNormal;
++            return world.worldProvider.isSkyMissing() ? world.paperConfig.lavaFlowSpeedNether : world.paperConfig.lavaFlowSpeedNormal;
 +        }
          if (this.material == Material.WATER && (
                  world.getType(blockposition.north(1)).getBlock().material == Material.LAVA ||
                          world.getType(blockposition.south(1)).getBlock().material == Material.LAVA ||
+diff --git a/src/main/java/net/minecraft/server/WorldProvider.java b/src/main/java/net/minecraft/server/WorldProvider.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/net/minecraft/server/WorldProvider.java
++++ b/src/main/java/net/minecraft/server/WorldProvider.java
+@@ -0,0 +0,0 @@ public abstract class WorldProvider {
+         return this.f;
+     }
+ 
++    public final boolean isSkyMissing() { return this.n(); } // Paper - OBFHELPER
+     public boolean n() {
+         return this.e;
+     }
 --