Fix WikitextContent::getSection() for missing sections

Fix WikitextContent::getSection() for missing sections

Make WikitextContent::getSection() return false when the section is
missing, as per the documented behaviour for Content::getSection(). Fix
a test so that it doesn't generate a PHP fatal error if getSection()
returns false.

Should fix the current Jenkins build failure:

https://integration.mediawiki.org/ci/job/MediaWiki-Tests-Misc/6989/testReport/junit/%28root%29/WikitextContentTest__testGetSection/testGetSection_with_data_set__2/

Change-Id: Ifa85f8eed50943d8ece32555d06b3e989077da46