[BUGFIX] Avoid PHP warning if there are no records to render for the method getRenderedRecords in AbstractContentViewHelper.

This little fix checks if $rows contains an array, so there will be no PHP warning if there're no records to render. This may be the case if there is an empty column in a backend layout.

 Core: Error handler (FE): PHP Warning: Invalid argument supplied for foreach() in .../typo3conf/ext/vhs/Classes/ViewHelpers/Content/AbstractContentViewHelper.php line 162

 The expected return value is still the same. If there are no records to render this would be an empty array.