Problem: Some modules are setting empty strings to extraConfig* options
with the intention to not generate any config. But empty
strings are also values, so they are still concatenated in the
final value of extraConfig* options. This results in a
multiple empty strings in extraConfigs.
Solution: Avoid using optionalString when setting values to extraConfig*
options. Use mkIf instead.
This commit also fixes mkIf condition in autocmd module.
`mkNeovimPlugin` is a special case. To avoid evaluating
caller's arguments mkMerge/optionalAttrs pattern is used
instead.
