Fix NPE in TestsInSuiteFunction.

Note that this only affects the new Skyframe-based implementation of the
LoadingPhaseRunner, which isn't live yet.

--
MOS_MIGRATED_REVID=114230017
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/TestsInSuiteFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/TestsInSuiteFunction.java
index 3c559de..4556c76 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/TestsInSuiteFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/TestsInSuiteFunction.java
@@ -116,6 +116,9 @@
       if (TargetUtils.isTestSuiteRule(suite)) {
         TestsInSuiteValue value =
             (TestsInSuiteValue) env.getValue(TestsInSuiteValue.key(suite, strict));
+        if (value == null) {
+          continue;
+        }
         builder.merge(value.getTargets());
       }
     }