[logviewer] Fix MessageView not being visible

Since 5.11.1 QWebEngineView::sizeHint() returns 0x0 instead of 800x600.
That results in zero width of the message view in the splitter.

See also: commit f8898170a77593e314d3cd187f259223e9bff0f8 in qtwebengine

Reviewers: KDE Telepathy

Reviewed By: davidedmundson

Subscribers: KDE Telepathy

Differential Revision: https://phabricator.kde.org/D19447