[fidlfmt] Fix bug in deleting blank lines.

Prior to this fix, if there were multiple blank lines following a
comment, the first one would get deleted because it needed to be
coalesced with the subsequent lines, and the subsequent lines would get
deleted because they were not next to a comment.  This would leave no
blank lines after a comment.

This was not caught by tests because we were only exercising it in the
case of an EOF.  In the EOF case, we don't want trailing newlines, so it
was the correct behavior.  The other cases where we had trailing newlines
after a comment (e.g., before const and enum) were special-cased to do the
right thing.

Change-Id: Ibba0e1ef7834cbe5a6e5af9c4e6faef837e724ea