Add checkerframework nullness annotations to accepted list for AnnotationTest (#5409)

Also use checkerframework annotations instead of jetbrains ones for Inventory#getContents as jetbrains ones do not allow properly annotating the nullability of an array and it's contents.

Spigot-API-Patches/Adventure.patch

@@ -3597,3 +3597,38 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      void setColor(@NotNull ChatColor color);
  
      /**
+diff --git a/src/test/java/org/bukkit/AnnotationTest.java b/src/test/java/org/bukkit/AnnotationTest.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/test/java/org/bukkit/AnnotationTest.java
++++ b/src/test/java/org/bukkit/AnnotationTest.java
+@@ -0,0 +0,0 @@ import org.objectweb.asm.tree.ParameterNode;
+ public class AnnotationTest {
+ 
+     private static final String[] ACCEPTED_ANNOTATIONS = {
++        // Paper start
++        "Lorg/checkerframework/checker/nullness/qual/Nullable;",
++        "Lorg/checkerframework/checker/nullness/qual/NonNull;",
++        "Lorg/checkerframework/checker/nullness/qual/PolyNull;",
++        "Lorg/checkerframework/checker/nullness/qual/MonotonicNonNull;",
++        // Paper end
+         "Lorg/jetbrains/annotations/Nullable;",
+         "Lorg/jetbrains/annotations/NotNull;",
+         "Lorg/jetbrains/annotations/Contract;",
+@@ -0,0 +0,0 @@ public class AnnotationTest {
+                     continue;
+                 }
+ 
+-                if (mustBeAnnotated(Type.getReturnType(method.desc)) && !isWellAnnotated(method.invisibleAnnotations)) {
++                if (mustBeAnnotated(Type.getReturnType(method.desc)) && !isWellAnnotated(method.invisibleAnnotations) && !isWellAnnotated(method.visibleTypeAnnotations)) { // Paper - also check visible type annotations
+                     warn(errors, clazz, method, "return value");
+                 }
+ 
+@@ -0,0 +0,0 @@ public class AnnotationTest {
+         return true;
+     }
+ 
+-    private static boolean isWellAnnotated(@Nullable List<AnnotationNode> annotations) {
++    private static boolean isWellAnnotated(@Nullable List<? extends AnnotationNode> annotations) { // Paper - allow children of AnnotationNode
+         if (annotations == null) {
+             return false;
+         }

Spigot-API-Patches/fix-Inventory-getContents-null-annotations.patch

@@ -15,7 +15,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 -    @NotNull
 -    public ItemStack[] getContents();
 -
-+    public @Nullable ItemStack @NotNull [] getContents(); // Paper - make array elements nullable instead array
++    public @org.checkerframework.checker.nullness.qual.Nullable ItemStack @org.checkerframework.checker.nullness.qual.NonNull [] getContents(); // Paper - make array elements nullable instead array
 +    
      /**
       * Completely replaces the inventory's contents. Removes all existing