[FIX] Zero Value is ignored in paperformat QWEB

**Zero Value is ignored in paperformat QWEB**

This Pull Request resolve #3367 

Impacted versions:

 - 8.0

Steps to reproduce:

 1. create a qweb report with paperformat.margin_left or right with Value Zero

Current behavior:

 - default margin is loaded (5 mm)

Expected behavior:

 - none margin