Fix incorrect nullability in MultipleFacing

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/block/data/MultipleFacing.java

@@ -3,7 +3,6 @@ package org.bukkit.block.data;
 import java.util.Set;
 import org.bukkit.block.BlockFace;
 import org.jetbrains.annotations.NotNull;
-import org.jetbrains.annotations.Nullable;
 
 /**
  * This class encompasses the 'north', 'east', 'south', 'west', 'up', 'down'
@@ -29,7 +28,7 @@ public interface MultipleFacing extends BlockData {
      * @param face to set
      * @param has the face
      */
-    void setFace(@Nullable BlockFace face, boolean has);
+    void setFace(@NotNull BlockFace face, boolean has);
 
     /**
      * Get all of the faces which are enabled on this block.