[SPARK-19876][SS] Follow up: Refactored BatchCommitLog to simplify logic

## What changes were proposed in this pull request?

Existing logic seemingly writes null to the BatchCommitLog, even though it does additional checks to write '{}' (valid json) to the log. This PR simplifies the logic by disallowing use of `log.add(batchId, metadata)` and instead using `log.add(batchId)`. No question of specifying metadata, so no confusion related to null.

## How was this patch tested?
Existing tests pass.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #17444 from tdas/SPARK-19876-1.