SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/command/ProxiedCommandSender.java

@@ -1,6 +1,8 @@
 
 package org.bukkit.command;
 
+import org.jetbrains.annotations.NotNull;
+
 public interface ProxiedCommandSender extends CommandSender {
 
     /**
@@ -8,6 +10,7 @@ public interface ProxiedCommandSender extends CommandSender {
      *
      * @return the caller which triggered the command
      */
+    @NotNull
     CommandSender getCaller();
 
     /**
@@ -15,6 +18,7 @@ public interface ProxiedCommandSender extends CommandSender {
      *
      * @return the caller which the command is being run as
      */
+    @NotNull
     CommandSender getCallee();
 
 }