getEntitySpecificJoins sometimes returns NULL, triggering deprecation warning for...

getEntitySpecificJoins sometimes returns NULL, triggering deprecation warning for trim() in php 8.1

Discovered this in the wild where an api3 Contact.getcount call for a particular smartgroup
was causing this. The smartgroup otherwise works fine.

Looking at the code, it's clear that getEntitySpecificJoins is supposed to return a string,
but in the case of civicrm_activity_contact it was returning from another function which may
return NULL. I have coalesced NULL to empty string.