Add more obviously missing @nullable annotations

retrofit-mock/src/main/java/retrofit2/mock/BehaviorCall.java

@@ -20,6 +20,7 @@
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.atomic.AtomicReference;
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.GuardedBy;
 import okhttp3.Request;
 import retrofit2.Call;
@@ -33,7 +34,7 @@ final class BehaviorCall<T> implements Call<T> {
   final ExecutorService backgroundExecutor;
   final Call<T> delegate;
 
-  private volatile Future<?> task;
+  private volatile @Nullable Future<?> task;
   volatile boolean canceled;
   @GuardedBy("this")
   private boolean executed;

retrofit-mock/src/main/java/retrofit2/mock/Calls.java

@@ -140,7 +140,7 @@ private static <T extends Throwable> T sneakyThrow2(Throwable t) throws T {
 
   static final class DeferredCall<T> implements Call<T> {
     private final Callable<Call<T>> callable;
-    private Call<T> delegate;
+    private @Nullable Call<T> delegate;
 
     DeferredCall(Callable<Call<T>> callable) {
       this.callable = callable;

retrofit-mock/src/main/java/retrofit2/mock/MockRetrofit.java

@@ -18,6 +18,7 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import javax.annotation.Nullable;
 import retrofit2.Retrofit;
 
 public final class MockRetrofit {
@@ -50,8 +51,8 @@ public <T> BehaviorDelegate<T> create(Class<T> service) {
 
   public static final class Builder {
     private final Retrofit retrofit;
-    private NetworkBehavior behavior;
-    private ExecutorService executor;
+    private @Nullable NetworkBehavior behavior;
+    private @Nullable ExecutorService executor;
 
     @SuppressWarnings("ConstantConditions") // Guarding public API nullability.
     public Builder(Retrofit retrofit) {