SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/util/FileUtil.java

@@ -1,5 +1,7 @@
 package org.bukkit.util;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.nio.channels.FileChannel;
 import java.io.File;
 import java.io.FileInputStream;
@@ -18,7 +20,7 @@ public class FileUtil {
      * @param outFile the target filename
      * @return true on success
      */
-    public static boolean copy(File inFile, File outFile) {
+    public static boolean copy(@NotNull File inFile, @NotNull File outFile) {
         if (!inFile.exists()) {
             return false;
         }