BaseTemplate: Handle `$this->data['language_urls'] === false` specially

Authored by matmarex on May 19 2017, 9:09 PM.

Description

BaseTemplate: Handle $this->data['language_urls'] === false specially

Per 53ab861a2fcba98d876e5a2be5af7bc7186b880c, extensions can set this
to an empty array to have the sidebar section displayed even if it has
no items. Wikidata and UniversalLanguageSelector rely on this.

Change-Id: I79cef25d79ed25fa6d96698a121c3105870e890b