Commit 59dbd43a19dd introduced a regression where setVisibleRegion(0, 0)
would expand to include the full line, causing
SegmentedModeTest.testShowNothing to fail. This change ensures that
empty ranges (length 0) are not expanded to the end of the line,
restoring the previous behavior for this case while keeping the fix for
partial lines (length > 0).

Co-authored-by: Lars Vogel <Lars.Vogel@vogella.com>