Fix computeAsync for empty keys

Signed-off-by: Yordan Grancharov <phrone@gmail.com>

redisson/src/main/java/org/redisson/RedissonMap.java

@@ -259,11 +259,8 @@ public class RedissonMap<K, V> extends RedissonExpirable implements RMap<K, V> {
                     return CompletableFuture.supplyAsync(() -> remappingFunction.apply(key, oldValue), getServiceManager().getExecutor())
                             .thenCompose(newValue -> {
                                 if (newValue != null) {
-                                    if (oldValue != null) {
+                                    return fastPutAsync(key, newValue)
-                                        return fastPutAsync(key, newValue)
+                                            .thenApply(rr -> newValue);
-                                                .thenApply(rr -> newValue);
-                                    }
-                                    return CompletableFuture.completedFuture(newValue);
                                 }
                                 return fastRemoveAsync(key)
                                         .thenApply(rr -> newValue);