Actually I couldn't find the code that triggers the above message when the username is not given. The default value for DBusername is given as root, so a null check wont be necessary. But how the message shows up is the error!

Inside mediawiki-core/includes/installer:

$ grep -R ". You have" *

Installer.i18n.php: 'config-mysql-old' => 'MySQL $1 or later is required. You have $2.',

Installer.i18n.php: 'config-postgres-old' => 'PostgreSQL $1 or later is required. You have $2.',

Installer.i18n.php: 'config-mysql-myisam-dep' => "'''Warning:''' You have selected MyISAM as storage engine for MySQL, which is not recommended for use with MediaWiki, because:

So you want to search for where "config-mysql-old" and other message keys are called (try looking for "config-" inside installer/Installer.php).

(In reply to comment #2)
after grepping in installer/ the only place where the message config-mysql-old comes out is
if ( version_compare( $version, $this->minimumVersion ) < 0 ) {

			return Status::newFatal( 'config-mysql-old', $this->minimumVersion, $version );
		}

but, strangely that comes out too when the username is kept null.

https://gerrit.wikimedia.org/r/#/c/118798/

Change 118798 had a related patch set uploaded by 01tonythomas:
Error Msg for missing db username & password when installing

https://gerrit.wikimedia.org/r/118798

Change 118798 merged by jenkins-bot:
Error Msg for missing db username & password when installing

https://gerrit.wikimedia.org/r/118798