GODRIVER-2577 Fix the logic of starting a heartbeat.

Author: qingyang-hu

x/mongo/driver/topology/server.go

@@ -735,13 +735,12 @@ func (s *Server) check() (description.Server, error) {
 	previousDescription := s.Description()
 	streamable := previousDescription.TopologyVersion != nil
 
-	isNilConn := s.conn == nil
-	if !isNilConn {
+	if s.conn != nil {
 		s.publishServerHeartbeatStartedEvent(s.conn.ID(), s.conn.getCurrentlyStreaming() || streamable)
 	}
 	// Create a new connection if this is the first check, the connection was closed after an error during the previous
 	// check, or the previous check was cancelled.
-	if isNilConn || s.conn.closed() || s.checkWasCancelled() {
+	if s.conn == nil || s.conn.closed() || s.checkWasCancelled() {
 		// Create a new connection and add it's handshake RTT as a sample.
 		err = s.setupHeartbeatConnection()
 		if err == nil {
@@ -751,9 +750,7 @@ func (s *Server) check() (description.Server, error) {
 		} else {
 			err = unwrapConnectionError(err)
 		}
-	}
-
-	if !isNilConn && err == nil {
+	} else {
 		// An existing connection is being used. Use the server description properties to execute the right heartbeat.
 
 		// Wrap conn in a type that implements driver.StreamerConnection.