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/RecursiveFilesystemTraversalFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
index df82357..eaed121 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
@@ -513,6 +513,7 @@
    *     {@link FileInfo} so the caller should use these instead of the old ones (this happens when
    *     a package is found, but under a different root than expected)
    */
+  @Nullable
   private static PkgLookupResult checkIfPackage(
       Environment env, TraversalRequest traversal, FileInfo rootInfo, SyscallCache syscallCache)
       throws IOException, InterruptedException, BuildFileNotFoundException {