Summary:
Allow `Slice` holding nullptr as a sentinel value but not in comparisons. This new restriction eliminates the need for the manual checks in 39ef900551a4d88c8546ca086baaba76730e6162, while still conforming to glibc's `memcmp` API. Thanks siying for the idea. Users may need to migrate, so mentioned it in HISTORY.md.
Closes https://github.com/facebook/rocksdb/pull/2777
Differential Revision: D5686016
Pulled By: ajkr
fbshipit-source-id: 03a2ca3fd9a0ebade9d0d5686c81d59a9534f563
234f33a3f9