[FIX] Fix the pdf download issue in tiki-print.php

• [FIX] Fix the pdf download issue in tiki-print.php The issue here is because we are trying to download the without checking if there is any pdf export mode installed or checked. if we go to http://localhost/tiki/tiki-print.php?page=HomePage&display=pdf without any module installed, the pdf will be invalid.

Regression n.52 : https://dev.tiki.org/Regressions-in-26x