En (em) dashes and spaces

First of all, a huge thank you for this tool!

I had a rather specific question regarding French typography. Shoudn’t en dashes – when used as “parenthetical remark” (_incises_) – be typeset with narrow no break spaces (or non breaking spaces, depending the mood :), like with french quotes ?
In the above sentence: « Shoudn’t[space]en[space]dashes[space]–[narrow no break space]when ».

Sometimes, em dashes are also used to mark incises (and maybe should be treated equally).
Unlike parenthesis, or quotes, en or em dashes don’t necessarily come by pairs, which might harden the possibility to guess what to do, not even saying the character is the same of opening and ending.

If you have any thoughts on this…
Thanks in advance.