fix(statusline): broken statusline on error #38000

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.

runtime/lua/vim/_meta/options.lua

@@ -6728,10 +6728,10 @@ vim.wo.stc = vim.wo.statuscolumn
 --- current window and buffer, while %{} items are evaluated in the
 --- context of the window that the statusline belongs to.
 ---
---- When there is error while evaluating the option then it will be made
---- empty to avoid further errors.  Otherwise screen updating would loop.
---- When the result contains unprintable characters the result is
---- unpredictable.
+--- When there is an error while evaluating the option it will be reset to
+--- its default value to avoid further errors.  Otherwise screen updating
+--- would loop.  When the result contains unprintable characters the
+--- result is unpredictable.
 ---
 --- Note that the only effect of 'ruler' when this option is set (and
 --- 'laststatus' is 2 or 3) is controlling the output of `CTRL-G`.