SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/UndefinedNullability.java

@@ -0,0 +1,26 @@
+package org.bukkit;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Annotation for types, whose nullability is not well defined, so
+ * {@link org.jetbrains.annotations.NotNull} nor
+ * {@link org.jetbrains.annotations.Nullable} is applicable. For example when
+ * interface defines a method, whose nullability depends on the implementation.
+ *
+ * @deprecated This should generally not be used in any new API code as it
+ * suggests a bad API design.
+ */
+@Retention(RetentionPolicy.CLASS)
+@Deprecated
+public @interface UndefinedNullability {
+
+    /**
+     * Human readable description of the circumstances, in which the type is
+     * nullable.
+     *
+     * @return description
+     */
+    String value() default "";
+}