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/ActionExecutionState.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionState.java
index 82c3dbc..d459173 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionState.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionExecutionState.java
@@ -79,6 +79,7 @@
     this.state = Preconditions.checkNotNull(state);
   }
 
+  @Nullable
   ActionExecutionValue getResultOrDependOnFuture(
       SkyFunction.Environment env,
       ActionLookupData actionLookupData,
@@ -133,6 +134,7 @@
     dummyFuture.set(null);
   }
 
+  @Nullable
   private ActionExecutionValue runStateMachine(SkyFunction.Environment env)
       throws ActionExecutionException, InterruptedException {
     ActionStepOrResult original;