Describe the bug
AST convert error

To Reproduce
Steps to reproduce the behavior:

1. select CONVERT(true, CHAR)
2. Parsing this SQL using JSqlParser with this statements
3. 

Expected behavior
why the key [true] and [Char] is instance of Column

System

• Database you are using
  mysql8.0
• Java Version
  11
• JSqlParser version
  4.2