Introduce Title::getTalkPageIfDefined.

Introduce Title::getTalkPageIfDefined.

This is part of the effort to remove the assumption that every page
can have a talk page. Before we can merge Icee208dc4 which makes
Title::getTalkPage() throw an exception of no corresponding talk
namespace is defined, all extensions that call getTalkPage() must
be changed to either check canHaveTalkPage() first, or to use
the conveniance function getTalkPageIfDefined() instead.

Bug: T165149
Bug: T172146
Change-Id: I6d2613d8f7105048022f8093186dc57f1f8173ab