Default implementation renders the Preview section from the template
file that the actual Provider returns for $row, using paths also
determined by $row. Example: fluidcontent's Provider returns files
and paths based on selected "Fluid Content type" and inherits and
uses this method to render a Preview from the template file in the
specific path. This default implementation expects the TYPO3 core
to render the default header, so it returns NULL as $headerContent.