[SPARK-7777][Streaming] Handle the case when there is no block in a batch

In the old implementation, if a batch has no block, `areWALRecordHandlesPresent` will be `true` and it will return `WriteAheadLogBackedBlockRDD`.

This PR handles this case by returning `WriteAheadLogBackedBlockRDD` or `BlockRDD` according to the configuration.

Author: zsxwing <zsxwing@gmail.com>

Closes #6372 from zsxwing/SPARK-7777 and squashes the following commits:

788f895 [zsxwing] Handle the case when there is no block in a batch