Remove 'premature optimization' that proved buggy

Remove 'premature optimization' that proved buggy

At some time it seemed like a good idea in breakRow() to return early
when the row was bound to be empty. It turns out that this creates two
symptoms:

* empty paragraphs will not have an end of paragraph marker

* since row width is not correctly computed in this case, caret ghosts
  can appear in master.

This commit removes the oprimization and replace the do {} while()
construct to a straightforward while() {}.

Related to bug #10952.

(cherry picked from commit 76f0a3dd4ee5c7958c6dea79628ef8c2ac2c59bd)