action=edit API with section=new would fail if page didn't exist

If the page doesn't exist, $articleObject->replaceSectionContent will return null
Fixes a regression introduced in be97167ab61e7d59d86f6a9dca53b7016d615735.

Bug: 52830
Change-Id: Icdc0f130eeb40fff5c656c396b2a2af294015b98
(cherry picked from commit 993de5fbd5dfde376f127f8e86c77c4f5972a722)