Fix: client reconnected every authenticationRefreshCheckSeconds when using tls authentication (#1062)

### Motivation

When using pulsar tls authentication with a broker that sets the authenticationRefreshCheckSeconds the connection was dropped for each authentication refresh check. After analyzing logs and tcpdumps I concluded that this error appears because the tls authentication is returning null, witch does not pass a validation in the broker.
After analyzing the tls auth implementation in Java (that works), I concluded that the GetData method should return empty byte array instead of nil.

### Modifications

Changed tls auth GetData to return empty byte array instead of nil.

---------

Co-authored-by: Jorge Pereira <jorge.pereira@cross-join.com>
(cherry picked from commit 16a0299)

Author: jffp113

pulsar/internal/connection.go

@@ -823,6 +823,11 @@ func (c *connection) handleAuthChallenge(authChallenge *pb.CommandAuthChallenge)
 		return
 	}
 
+	// Brokers expect authData to be not nil
+	if authData == nil {
+		authData = []byte{}
+	}
+
 	cmdAuthResponse := &pb.CommandAuthResponse{
 		ProtocolVersion: proto.Int32(PulsarProtocolVersion),
 		ClientVersion:   proto.String(ClientVersionString),