Add empty fission yields to tests.TestChain

This object is used in unit testing the depletion schemes, where
the depletion matrix is pre-defined and there is no
need for fission yields. However, the depletion interface
expects the chain to have a fission_yields attribute.

tests/dummy_operator.py

@@ -7,6 +7,13 @@ from openmc.deplete.abc import TransportOperator, OperatorResult
 
 
 class TestChain(object):
+    """Empty chain to assist with unit testing depletion routines
+
+    Only really provides the form_matrix function, but acts like
+    a real Chain
+    """
+
+    fission_yields = [None]
 
     @staticmethod
     def get_thermal_fission_yields():