Annotate ValueWithMetadata#justValue as @Nullable.

PiperOrigin-RevId: 211116899
diff --git a/src/main/java/com/google/devtools/build/skyframe/ValueWithMetadata.java b/src/main/java/com/google/devtools/build/skyframe/ValueWithMetadata.java
index 805029a..d7b53c8 100644
--- a/src/main/java/com/google/devtools/build/skyframe/ValueWithMetadata.java
+++ b/src/main/java/com/google/devtools/build/skyframe/ValueWithMetadata.java
@@ -257,6 +257,7 @@
     }
   }
 
+  @Nullable
   public static SkyValue justValue(SkyValue value) {
     if (value instanceof ValueWithMetadata) {
       return ((ValueWithMetadata) value).getValue();