webmail: don't have two spaces between header and address(es) (e.g. for From/To)

because outlook.com will reformat the message and then fail to verify the message.
proton.me also reformats and invalidates the dkim signature, but seemingly
after it verifies the dkim signature.

webmail/api.go

@@ -391,7 +391,7 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 			if v != "" && linelen+1+len(s) > 77 {
 				v += "\r\n\t"
 				linelen = 1
-			} else {
+			} else if v != "" {
 				v += " "
 				linelen++
 			}