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.

src/nvim/options.lua

@@ -8794,10 +8794,10 @@ local options = {
         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|.