Refine CcFromString to handle spaces and empty addresses.

This change ensures that email addresses with leading or trailing spaces are trimmed, and empty addresses resulting from multiple commas are ignored. This helps prevent potential errors with malformed email addresses in the "Cc" field.

msg.go

@@ -847,7 +847,16 @@ func (m *Msg) CcIgnoreInvalid(rcpts ...string) {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.3
 func (m *Msg) CcFromString(rcpts string) error {
-	return m.Cc(strings.Split(rcpts, ",")...)
+	src := strings.Split(rcpts, ",")
+	var dst []string
+	for _, address := range src {
+		address = strings.TrimSpace(address)
+		if address == "" {
+			continue
+		}
+		dst = append(dst, address)
+	}
+	return m.Cc(dst...)
 }
 
 // Bcc sets one or more "BCC" (blind carbon copy) addresses in the mail body for the Msg.