Add nil check for smtpClient in checkConn function

Previously, if smtpClient was nil, the checkConn function would not handle that case. This update ensures that an appropriate error is returned when smtpClient is nil, enhancing the robustness of the client connection checks.

client.go

@@ -1298,6 +1298,9 @@ func (c *Client) sendSingleMsg(message *Msg) error {
 //   - An error if there is no active connection, if the NOOP command fails, or if extending
 //     the deadline fails; otherwise, returns nil.
 func (c *Client) checkConn() error {
+	if c.smtpClient == nil {
+		return ErrNoActiveConnection
+	}
 	if !c.smtpClient.HasConnection() {
 		return ErrNoActiveConnection
 	}