WebTab: Fix tab with empty url but not empty history treating as invalid

Fixes possibility to lose tabs on session restore when closing window
right after it started to restore session (right after it was opened).

src/lib/webtab/webtab.cpp

@@ -56,7 +56,7 @@ WebTab::SavedTab::SavedTab(WebTab* webTab)
 
 bool WebTab::SavedTab::isValid() const
 {
-    return !url.isEmpty();
+    return !url.isEmpty() || !history.isEmpty();
 }
 
 void WebTab::SavedTab::clear()