fix(options): better handling of empty values

Problem:

Whether an option is allowed to be empty isn't well defined and
isn't properly checked.

Solution:

- For non-list string options, explicitly check the option value
  if it is empty.
- Annotate non-list string options that can accept an empty value.
  - Adjust command completion to ignore the empty value.
- Render values in Lua meta files

scripts/gen_eval_files.lua

@@ -666,7 +666,16 @@ local function render_option_meta(_f, opt, write)
     write('--- ' .. l)
   end
 
-  write('--- @type ' .. OPTION_TYPES[opt.type])
+  if opt.type == 'string' and not opt.list and opt.values then
+    local values = {} --- @type string[]
+    for _, e in ipairs(opt.values) do
+      values[#values + 1] = fmt("'%s'", e)
+    end
+    write('--- @type ' .. table.concat(values, '|'))
+  else
+    write('--- @type ' .. OPTION_TYPES[opt.type])
+  end
+
   write('vim.o.' .. opt.full_name .. ' = ' .. render_option_default(opt.defaults))
   if opt.abbreviation then
     write('vim.o.' .. opt.abbreviation .. ' = vim.o.' .. opt.full_name)