compose: prevent sending empty address list headers

Aerc was sending empty address list header fields (specifically CC by
default). This was causing DKIM failures in lists.sr.ht. RFC 5322 states
that an address field should consist of the field name and one or more
addresses, implying empty fields are not allowed.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

widgets/compose.go

@@ -952,6 +952,10 @@ func (he *headerEditor) storeValue() {
 	val := he.input.String()
 	switch strings.ToLower(he.name) {
 	case "to", "from", "cc", "bcc":
+		if strings.TrimSpace(val) == "" {
+			// Don't set empty address list headers
+			return
+		}
 		list, err := mail.ParseAddressList(val)
 		if err == nil {
 			he.header.SetAddressList(he.name, list)