Add missing nullability annotations

Author: lukas-krecan

providers/mongo/shedlock-provider-mongo-reactivestreams/src/main/java/net/javacrumbs/shedlock/provider/mongo/reactivestreams/ReactiveStreamsMongoLockProvider.java

@@ -26,6 +26,7 @@
 import net.javacrumbs.shedlock.core.SimpleLock;
 import net.javacrumbs.shedlock.support.LockException;
 import net.javacrumbs.shedlock.support.Utils;
+import net.javacrumbs.shedlock.support.annotation.Nullable;
 import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.reactivestreams.Publisher;
@@ -157,6 +158,7 @@ private void unlock(LockConfiguration lockConfiguration) {
         ));
     }
 
+    @Nullable
     static <T> T execute(Publisher<T> command) {
         SingleLockableSubscriber<T> subscriber = new SingleLockableSubscriber<>();
         command.subscribe(subscriber);

providers/mongo/shedlock-provider-mongo-reactivestreams/src/main/java/net/javacrumbs/shedlock/provider/mongo/reactivestreams/SingleLockableSubscriber.java

@@ -58,10 +58,12 @@ public void onComplete() {
         latch.countDown();
     }
 
+    @Nullable
     T getValue() {
         return value;
     }
 
+    @Nullable
     Throwable getError() {
         return error;
     }