Problem: after #33036, an error from evaluating 'statusline' clears it and
doesn't draw the statusline. (causing glitchy redraws)
Solution: use the default value instead. If 'stl' is somehow ever empty, still
call redraw_custom_statusline to at least draw an empty statusline.
Ideally our default 'stl' shouldn't itself error too! :-)
Also adjust some prior screen:expect()s to avoid immediate success warnings.
32d3dd0650
