SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/conversations/Prompt.java

@@ -1,5 +1,8 @@
 package org.bukkit.conversations;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
 /**
  * A Prompt is the main constituent of a {@link Conversation}. Each prompt
  * displays text to the user and optionally waits for a user's response.
@@ -21,7 +24,8 @@ public interface Prompt extends Cloneable {
      * @param context Context information about the conversation.
      * @return The text to display.
      */
-    String getPromptText(ConversationContext context);
+    @NotNull
+    String getPromptText(@NotNull ConversationContext context);
 
     /**
      * Checks to see if this prompt implementation should wait for user input
@@ -29,9 +33,9 @@ public interface Prompt extends Cloneable {
      *
      * @param context Context information about the conversation.
      * @return If true, the {@link Conversation} will wait for input before
-     *     continuing.
+     *     continuing. If false, {@link #acceptInput(ConversationContext, String)} will be called immediately with {@code null} input.
      */
-    boolean blocksForInput(ConversationContext context);
+    boolean blocksForInput(@NotNull ConversationContext context);
 
     /**
      * Accepts and processes input from the user. Using the input, the next
@@ -41,5 +45,6 @@ public interface Prompt extends Cloneable {
      * @param input The input text from the user.
      * @return The next Prompt in the prompt graph.
      */
-    Prompt acceptInput(ConversationContext context, String input);
+    @Nullable
+    Prompt acceptInput(@NotNull ConversationContext context, @Nullable String input);
 }