composer: remove header if empty

Remove a header when it is empty.

Commit a253e89bda ("compose: prevent sending empty address list
headers") tried to avoid sending empty headers; but instead of deleting
the header, the empty string value was just ignored.

Fixes: https://todo.sr.ht/~rjarry/aerc/55
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

widgets/compose.go

@@ -974,7 +974,8 @@ func (he *headerEditor) storeValue() {
 	switch strings.ToLower(he.name) {
 	case "to", "from", "cc", "bcc":
 		if strings.TrimSpace(val) == "" {
-			// Don't set empty address list headers
+			// if header is empty, delete it
+			he.header.Del(he.name)
 			return
 		}
 		list, err := mail.ParseAddressList(val)