Fix computeAsync for empty keys

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

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

@@ -259,12 +259,9 @@ 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)
                                             .thenApply(rr -> newValue);
                                 }
-                                    return CompletableFuture.completedFuture(newValue);
-                                }
                                 return fastRemoveAsync(key)
                                         .thenApply(rr -> newValue);
                             });