• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

environment

• canal 1.1.6

Issue Description

在不填cluster的时候，拼接url时会出现cluster="null"。

Steps to reproduce

应设置默认值""或判空设为"".

Expected behaviour

正确拼接应该是cluster=""。

Actual behaviour

cluster="null"

If there is an exception, please attach the exception trace:

Just put your stack trace here!