Manually NullMark usage_stats files

Bug: 389129271
Change-Id: I5bc7912d9135615225da2fa2ebcd143326cc647c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6829047
Reviewed-by: Henrique Nakashima <[email protected]>
Owners-Override: Henrique Nakashima <[email protected]>
Commit-Queue: Jenna Himawan <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1500415}
NOKEYCHECK=True
GitOrigin-RevId: 80f39c2d61a73ae93787d11e686ecd67cba01b01

Author: jenna-h

android/java/src/org/chromium/base/CallbackUtils.java

@@ -5,6 +5,7 @@
 package org.chromium.base;
 
 import org.chromium.build.annotations.NullMarked;
+import org.chromium.build.annotations.Nullable;
 
 /** Utilities for interacting with {@link Callback}s. */
 @NullMarked
@@ -19,7 +20,7 @@ public class CallbackUtils {
 
     /** Returns a Singleton {@link Callback} to be used where you need no action to be taken. */
     @SuppressWarnings("unchecked")
-    public static <T> Callback<T> emptyCallback() {
+    public static <T extends @Nullable Object> Callback<T> emptyCallback() {
         return DO_NOTHING_CALLBACK;
     }