Fix ghost caret

Fix ghost caret

This fixes a regression in e64ea357, where a test has been (badly)
tightened to avoid that two consecutive rows may be redrawn to get rid
of caret ghosts. The test prohibited empty rows from being redrawn.

Moreover, improve the test of cursor boundary to avoid the case where
cursor position is already 0.

Fixes bug #10952.

(cherry picked from commit 66c677b9463feb0687a8228603f86eddd4e859fd)