Add null check to AsyncItemWriter

The AsyncItemWriter is used in conjunction with the AsyncItemProcessor
to unwrap the Futures that are returned by that processor.
Traditionally when an ItemProcessor returns null, it's considered having
been filtered out and should not be passed to the ItemWriter.  In this
case, the AsyncItemWriter was not checking for nulls so they were being
passed to the delegate ItemWriter.  Most of the OOTB ItemWriters do not
perform a null check prior to doing the write so they were throwing NPEs
when using this paradigm.