Add flag to allow baseline files to be empty

Normally baselines are deleted if they are empty. This
CL adds a flag to let you leave baselines in place even
if they are empty.

--delete-empty-baselines  Whether to delete baseline files if they are
                          updated and there is nothing to include.

Also tweak baselines to not end with two blank lines.

Test: make update-api
Change-Id: I7c24d6ac531ad32dfd0e45e2ebd3f2d4c7591338