Add missing `@Nullable` annotation for method that returns `null` in its default implementation

PiperOrigin-RevId: 374394667
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
index ee8a937b..b01b14f 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
@@ -40,6 +40,7 @@
   /** All <code>ConfiguredTarget</code>s have a "files" field. */
   String FILES_FIELD = "files";
 
+  @Nullable
   default String getConfigurationChecksum() {
     return getConfigurationKey() == null ? null : getConfigurationKey().getOptions().checksum();
   }