Viewers not working when plotting meshes with zero cells in parallel

[wd15]

mpirun -np 16 python examples/diffusion/mesh20x20.py --trilinos

gives a load of these types of failures

mportError: Failed to import a viewer: [min() arg is an empty sequence']('matplotlib:)

for example. Issues noticed by Farenc Tasnadi
http://thread.gmane.org/gmane.comp.python.fipy/2341.
This a bit of an edge case, but worth fixing for robustness.

Imported from trac ticket #418, created by wd15 on 01-04-2012 at 11:58, last modified: 01-31-2012 at 16:48

[guyer]

With r5147, the Viewer factory returns a DummyViewer for all variables that contain no cells.

Trac comment by guyer on 01-31-2012 at 16:48