Summary:
This is a followup to #4311. Checking `!RangeDelAggregator::IsEmpty()` before opening a dedicated range tombstone SST did not properly prevent empty SSTs from being generated. That's because it relies on `CollapsedRangeDelMap::Size`, which had an underflow bug when the map was empty. This PR fixes that underflow bug.

Also fixed an uninitialized variable in db_stress.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4336

Differential Revision: D9600080

Pulled By: ajkr

fbshipit-source-id: bc6980ca79d2cd01b825ebc9dbccd51c1a70cfc7