Ignore a zero-sized file while looking for a seq-no in GetUpdatesSince

Ignore a zero-sized file while looking for a seq-no in GetUpdatesSince

Summary:
Rocksdb can create 0 sized log files when it is opened and closed without any operations.
The GetUpdatesSince fails currently if there is a log file of size zero.

This diff fixes this. If there is a log file is 0, it is removed form the probable_file_list

Test Plan: unit test

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

CC: leveldb
Differential Revision: https://reviews.facebook.net/D9507