[FIX][UX] tiki-webmail.php : force a wide th

In cypht standalone the same html is displaying correctly with a column

header text being empty. In tiki the column is getting totally hidden probalty due to css conflict between integration, so this simplest tweak force an empty space on the header to allow it's content being displayed alongside the table content.

Related to : https://avan.tech/item83792-Cypht-profile-edit-button-missing