[SPARK-12136][STREAMING] rddToFileName does not properly handle prefix and suffix parameters

The original code does not properly handle the cases where the prefix is null, but suffix is not null - the suffix should be used but is not.

The fix is using StringBuilder to construct the proper file name.

Author: bomeng <bmeng@us.ibm.com>
Author: Bo Meng <mengbo@bos-macbook-pro.usca.ibm.com>

Closes #10185 from bomeng/SPARK-12136.