SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/attribute/AttributeInstance.java

@@ -1,5 +1,7 @@
 package org.bukkit.attribute;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.Collection;
 
 /**
@@ -13,6 +15,7 @@ public interface AttributeInstance {
      *
      * @return the attribute
      */
+    @NotNull
     Attribute getAttribute();
 
     /**
@@ -34,6 +37,7 @@ public interface AttributeInstance {
      *
      * @return a copied collection of all modifiers
      */
+    @NotNull
     Collection<AttributeModifier> getModifiers();
 
     /**
@@ -41,14 +45,14 @@ public interface AttributeInstance {
      *
      * @param modifier to add
      */
-    void addModifier(AttributeModifier modifier);
+    void addModifier(@NotNull AttributeModifier modifier);
 
     /**
      * Remove a modifier from this instance.
      *
      * @param modifier to remove
      */
-    void removeModifier(AttributeModifier modifier);
+    void removeModifier(@NotNull AttributeModifier modifier);
 
     /**
      * Get the value of this instance after all associated modifiers have been