Fix the failure of stringappend_test caused by PartialMergeMulti.

Summary:
Fix a bug that PartialMergeMulti will try to merge the first operand
with an empty slice.

Test Plan: run stringappend_test and merge_test.

Reviewers: haobo, igor

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17157