DDC-3159: CONCAT expression for PostGreSql

[doctrinebot]

Jira issue originally created by user maximecolin:

For PostGreSQL, the CONCAT DQL function is translated in concatenation with || operator (which is the default behavior in AbstractPlatform class).

Is there a particular reason to not use the CONCAT PostGreSQL function instead like in MySqlPlatform ?

I ask this cause the concatenation with || operator return null if one of the part is null, whereas CONCAT function will simply ignore null values.