Do not save empty mails as draft

[odzhychko]

Is your feature request related to a problem? Please describe.

Currently, empty drafts get saved littering the draft folder.

Reproduce:

• Start writing a new mail (click "+ New message")
• Do nothing (no changes, no modifications)
• Close editor
• Empty draft is saved

Describe the solution you'd like

If the Mail is empty, do not save it as draft.
A mail is empty, if:

• has no subject
• has no recipient
• has no attachments
• body is empty or only has the automatic signature

Describe alternatives you've considered

No response

Additional context

No response