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/TestCompletionFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/TestCompletionFunction.java
index f2357ef..8b482fa 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/TestCompletionFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/TestCompletionFunction.java
@@ -24,6 +24,7 @@
 import com.google.devtools.build.skyframe.SkyFunction;
 import com.google.devtools.build.skyframe.SkyKey;
 import com.google.devtools.build.skyframe.SkyValue;
+import javax.annotation.Nullable;
 
 /**
  * TestCompletionFunction builds all relevant test artifacts of a {@link
@@ -32,6 +33,7 @@
  */
 public final class TestCompletionFunction implements SkyFunction {
   @Override
+  @Nullable
   public SkyValue compute(SkyKey skyKey, Environment env) throws InterruptedException {
     TestCompletionValue.TestCompletionKey key =
         (TestCompletionValue.TestCompletionKey) skyKey.argument();