SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/projectiles/BlockProjectileSource.java

@@ -1,6 +1,7 @@
 package org.bukkit.projectiles;
 
 import org.bukkit.block.Block;
+import org.jetbrains.annotations.NotNull;
 
 public interface BlockProjectileSource extends ProjectileSource {
 
@@ -9,5 +10,6 @@ public interface BlockProjectileSource extends ProjectileSource {
      *
      * @return Block for the projectile source
      */
+    @NotNull
     public Block getBlock();
 }