allow omission of diffCoeffModel in couplingProperties dict

if 'diffCoeffModel' is not defined in couplingProperties, create an
instance of noDiffCoeff ("off") and also do not try to read 'D' from
transportProperties dict

src/lagrangian/cfdemParticle/subModels/diffCoeffModel/diffCoeffModel/diffCoeffModel.C

@@ -73,7 +73,7 @@ diffCoeffModel::diffCoeffModel
     )
 {
     // build constant fields for single phase case
-    if (!particleCloud_.multiphase())
+    if (!particleCloud_.multiphase() && dict_.found("diffCoeffModel"))
     {
         D0Field_ = volScalarField
         (

src/lagrangian/cfdemParticle/subModels/diffCoeffModel/diffCoeffModel/newDiffCoeffModel.C

@@ -37,7 +37,7 @@ autoPtr<diffCoeffModel> diffCoeffModel::New
 {
     word diffCoeffModelType
     (
-        dict.lookup("diffCoeffModel")
+        dict.lookupOrDefault<word>("diffCoeffModel","off")
     );
 
     Info<< "Selecting diffCoeffModel "