Add checking value is null in declareDependenciesAndCheckValues.

Return earlier if any value is missing, don't need to iterate over.

PiperOrigin-RevId: 436189630

Author: emilyguo1

src/main/java/com/google/devtools/build/lib/skyframe/BuildDriverFunction.java

@@ -218,7 +218,9 @@ private static void declareDependenciesAndCheckValues(
       Environment env, Iterable<? extends SkyKey> skyKeys) throws InterruptedException {
     SkyframeIterableResult result = env.getOrderedValuesAndExceptions(skyKeys);
     while (result.hasNext()) {
-      result.next();
+      if (result.next() == null) {
+        return;
+      }
     }
   }