An exception is raised when a page is deleted in a farm where not all the sub-wikis have the extension installed

Steps to reproduce:

• create two sub-wikis: wiki1 and wiki2
• install extension in wiki1
• delete page from wiki2

Expected result:

• the page is deleted without error

Actual result:

• the delete listener of the extension is called and raises an error due to an XQWL query which assumes that the extension is installed in the wiki the delete originates from