fix(GODT-1804): Only promote content headers if non-empty

When attaching public key, we take the root mime part, create a new root,
and put the old root alongside an additional public key mime part.
But when moving the root, we would copy all content headers, even empty ones.
So we’d be left with Content-Disposition: "" which would fail to parse.

internal/user/smtp.go

@@ -120,7 +120,7 @@ func (user *User) sendMail(authID string, from string, to []string, r io.Reader)
 			}
 
 			// If we have to attach the public key, do it now.
-			if settings.AttachPublicKey == proton.AttachPublicKeyEnabled {
+			if settings.AttachPublicKey {
 				key, err := addrKR.GetKey(0)
 				if err != nil {
 					return fmt.Errorf("failed to get sending key: %w", err)