Summary: DBTest.DynamicLevelMaxBytesCompactRange needs to make sure L0 is not empty to properly cover the code paths we want to cover. However, current codes have a bug that might leave the condition not held. Improve the test to ensure it.

Test Plan: Run the test in an environment that is used to fail. Also run it many times.

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D38631