Revision 2289

* test/test_viewport.py
(ViewPortTest.test_changing_map_projection): Check that changing
the projection of an empty map shown in a viewport doesn't lead to
exceptions in the viewport's handler for the
MAP_PROJECTION_CHANGED messages

* Thuban/UI/viewport.py (ViewPort.map_projection_changed): Only
try to keep the same region visible when the map actually contains
something