Avoid possible null pointer dereference in WebEnginePartDownloadManager::blobDownloadedToFile()

In the 'else' block, the 'page' pointer must be null because a non-null
pointer is handled in the 'if' part.  If this block were to be executed,
there would be a null dereference through the pointer.  GCC detects this
with:

webenginepart/src/webenginepartdownloadmanager.cpp: In member function
  'void WebEnginePartDownloadManager::blobDownloadedToFile(QWebEngineDownloadItem*, WebEnginePage*)':
webenginepart/src/webenginepartdownloadmanager.cpp:177: warning: 'this' pointer is null
/usr/include/qt5/QtWebEngineWidgets/qwebenginepage.h:244: note: in a call to non-static
  member function 'QWidget* QWebEnginePage::view() const'

Since 'w' can never be a valid widget, the solution is simply to pass
nullptr to the JobUiIDelegate.