Don't fail if there are no comments.

Grep exits with non-zero if it find no matches. But it doesn't matter in
this case, so just carry on.

Test: frameworks/base/tools/hiddenapi/sort_api.sh \
Test:    frameworks/base/config/hiddenapi-light-greylist.txt
Change-Id: I7c219ee617f7dfa3cff5232ab979cc4ba38ebbfd