Velocity error logged on every page view when a docextra UI extension doesn't define the itemnumber parameter

Steps to reproduce

1. Create a page with an XWiki.UIExtensionClass object on the extension point org.xwiki.plaftorm.template.docextra, scope wiki, with the following parameters (note: no itemnumber):

   show=true
   title=Test
   name=test
   

2. View any page.

Expected

The new tab is displayed at the bottom of the page without an item count, and nothing is logged. The extension point documentation lists itemnumber as optional.

Actual

The tab is displayed correctly, but every page view logs an error in the server logs, once per such tab:

ERROR o.a.v.rendering - Left side ($docextra.get(3)) of comparison operation has null value at environment:/skins/flamingo/docextra.vm[line 95, column 55]

The item count of a UI extension tab is taken from $params.itemnumber, which is null when the parameter is missing, and docextra.vm then compares it with >= 0.

This also happens in the functional tests: the tabs created by DocExtraTabsIT don't define itemnumber, so every page view of the tests that run after it in xwiki-platform-flamingo-skin-test-docker logs this error.