Summary:
Relax the check condition of prefix_extractor in CheckOptionsCompatibility
by allowing changing value from non-nullptr to nullptr or nullptr to
non-nullptr.
Test Plan:
options_test
options_util_test
Reviewers: sdong, anthony, IslamAbdelRahman, kradhakrishnan, gunnarku
Reviewed By: gunnarku
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D54477