QtHelp page loading fix
Summary: This patch fixes processing of empty requests, which is necessary for successful loadFinished() signal emitting by corresponding QWebView.
Test Plan: Tested on master branch with Qt 5.7.0
Reviewers: KDevelop, kfunk
Subscribers: kfunk, kdevelop-devel
Differential Revision: https://phabricator.kde.org/D3671