Fix crash when restoring a session stored with Dolphin 4.13 or earlier

Fix crash when restoring a session stored with Dolphin 4.13 or earlier

Since DolphinTabPage::saveState() and
DolphinTabPage::restoreState(const QByteArray& state) save and restore
the state of each tab in a different format than DolphinMainWindow did
before the refactoring, we can run into problems: the first time a user
logs into a session that has Dolphin 4.14, Dolphin might read session
data that does not contain the QByteArray that DolphinTabPage wants to
read the data from.

In restoreState, isSplitViewEnabled will thus have the value false, and
no secondary view will be created. Later on, m_primaryViewActive will
also be set to false, but the else branch of the following
"if (m_primaryViewActive)" then tries to activate the secondary view,
which does not exist -> we get a crash.

The easiest solution is to not restore the tab state if no session data
in the new format is found.

BUG: 338187
REVIEW: 119718
FIXED-IN: 4.14.0