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

test/unit/optionstr_spec.lua

@@ -11,8 +11,8 @@ local check_ff_value = function(ff)
 end
 
 describe('check_ff_value', function()
-  itp('views empty string as valid', function()
-    eq(1, check_ff_value(''))
+  itp('views empty string as invalid', function()
+    eq(0, check_ff_value(''))
   end)
 
   itp('views "unix", "dos" and "mac" as valid', function()