Several part of the code pass false and not null to sourceFile.
For example, in includes/installer/DatabaseUpdater.php, we've:
Sample of code to update
public function copyFile( $filename ) { $this->db->sourceFile( $filename, false, false, false, [ $this, 'appendLine' ] ); }
New signature of Database::sourceFile method
public function sourceFile( $filename, callable $lineCallback = null, callable $resultCallback = null, $fname = false, callable $inputCallback = null );