provide "shape" and "format" to empty Qsymm models as required by 1.2

Not providing these is now deprecated.
Close #325.

kwant/qsymm.py

@@ -68,8 +68,8 @@ def builder_to_model(syst, momenta=None, real_space=True,
     is used in finalized kwant systems.
     """
     def term_to_model(d, par, matrix):
-        if np.allclose(matrix, 0):
-            result = BlochModel({})
+        if allclose(matrix, 0):
+            result = BlochModel({}, shape=matrix.shape, format=np.ndarray)
         else:
             result = BlochModel({BlochCoeff(d, qsymm.sympify(par)): matrix},
                                 momenta=momenta)