Fix full screen view when all pages deleted

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.windows.plotwindow", line 1245, in resizeEvent
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.document.doc", line 535, in pageSize
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.document.doc", line 522, in _pagedocsize
AttributeError: 'NoneType' object has no attribute 'settings'