feat(admin): check whether notification mail text filled

Description

feat(admin): check whether notification mail text filled

• do not allow to set a notification as enabled without at least one localized text filled

How to test

Try to create/edit empty mail notifications (e.g. here http://localhost:4200/organizations/3987/settings/notifications)

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Other information

This is related to the issue of sending empty emails if the notification is set as enabled, but all the formats/languages are left emtpy

Related issues

re STR-1199