ENH: Radiation model - use subOrEmptyDict for sib-model coeffs

src/thermophysicalModels/radiationModels/radiationModel/radiationModel/radiationModel.C

@@ -119,7 +119,7 @@ Foam::radiation::radiationModel::radiationModel
     time_(T.time()),
     T_(T),
     radiation_(lookup("radiation")),
-    coeffs_(subDict(type + "Coeffs")),
+    coeffs_(subOrEmptyDict(type + "Coeffs")),
     solverFreq_(lookupOrDefault<label>("solverFreq", 1)),
     firstIter_(true),
     absorptionEmission_(absorptionEmissionModel::New(*this, mesh_)),
@@ -152,7 +152,7 @@ Foam::radiation::radiationModel::radiationModel
     time_(T.time()),
     T_(T),
     radiation_(lookup("radiation")),
-    coeffs_(subDict(type + "Coeffs")),
+    coeffs_(subOrEmptyDict(type + "Coeffs")),
     solverFreq_(lookupOrDefault<label>("solverFreq", 1)),
     firstIter_(true),
     absorptionEmission_(absorptionEmissionModel::New(*this, mesh_)),