Add more unit test for `SQLStats`

[strongduanmu]

Hi community,

This issue is for https://github.com/apache/shardingsphere/issues/20305.

Aim

Add more unit tests for SQLStats to test its public functions to improve unit test coverage.

Unit test coverage can be viewed at: https://app.codecov.io/gh/apache/shardingsphere/blobs/master/shardingsphere-sql-parser/shardingsphere-sql-parser-statement/src/main/java/org/apache/shardingsphere/sql/parser/sql/common/SQLStats.java

Basic Qualifications

• Java
• Maven
• Junit