Avoid fatal in Article::fetchContent()

The function is documented as returning false if there is no content to
the page, but this logic was lost in the ContentHandler change.

Even though it's "BC cruft", it still shouldn't fatal for something as
simple as a non-existent page.

Bug: 43737
Change-Id: Ic1924a42fa7b4227ac65838d0def6a66c3990573
(cherry picked from commit 26d114090a04fc589cd00aa4432407b3e1ce01f5)
Merge "Avoid fatal in Article::fetchContent()" into wmf/1.24wmf2