explicit engine: Properly signal "not supported" for MDP total reward computations

Previously,
prism prism-examples/dice/two_dice.nm -pf 'Rmax=?[C]' -ex
would yield a NullPointerException.


git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@11522 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/explicit/ProbModelChecker.java

@@ -1024,7 +1024,6 @@ public class ProbModelChecker extends NonProbModelChecker
 			res = ((CTMCModelChecker) this).computeTotalRewards((CTMC) model, (MCRewards) modelRewards);
 			break;
 		case MDP:
-			break;
 		default:
 			throw new PrismNotSupportedException("Explicit engine does not yet handle the " + expr.getOperatorSymbol() + " reward operator for " + model.getModelType()
 					+ "s");