Add @Nullable annotations where they were missing.

PiperOrigin-RevId: 459211611
Change-Id: I9b281b82c8b3356b3ec735752d9da93239d90105
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/AspectFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/AspectFunction.java
index bba79c9..4d82e0b 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/AspectFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/AspectFunction.java
@@ -720,6 +720,7 @@
         transitivePackagesForPackageRootResolution);
   }
 
+  @Nullable
   private static AspectValue createAliasAspect(
       Environment env,
       Target originalTarget,