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

(cherry picked from commit 2c04b1a85078fb0aa369f1e3e6113787cbe80268)