Remove unnecessary expressions in AsyncItemWriter

[ParadiseCHOI]

1. Modify unnecessary parameter expression, list.add(future.get()) to list.add(item)
2. Modify unnecessary conditional expression, if (cause != null && cause instanceof Exception) to if (cause instanceof Exception)
3. Add conditional expression when list is empty

1. Unwrapped future's value have alreay been assigned to the item variable. So, 'item' variable should be delivered instead of future.get() in the list.add method.

2. The getCause method of the Throwable Object returns null if the cause is nonexistent or unknown. And instanceOf operator always returns false if the object is null. So, 'cause != null' expression is unnecessary.

3. While looking at Future related to asynchronous processing, I found that it was in use in AsyncItem (Processor, Writer) of spring-batch.
Looking at the internal implementation, it was written in violation of the processor's default behavior, null return, so I modified the code so that if items are empty in AsyncItemWriter, nothing is returned.

[fmbenhassine]

What to do with empty chunks is a decision to be made by the delegate writer, not the asynchronous item writer. A concrete example is that when the delegate logs a warning about receiving an empty chunk, with this change, the log message would disappear. The role of the async item writer is to unwrap Futures objects and pass them to the delegate writer without any modification or additional logic. I will omit this change from the PR.

[fmbenhassine]

Rebased and merged as 48925f0. Thank you for your contribution!