Fix NPE in fast adaptive uniformisation (cumulative reward, transition rewards without action name)

The action object attached to a transition can be null (internal action), leading to a null pointer
exception when trying to call the toString method.

+ test case

prism-tests/bugfixes/ctmc-fau-npe.sm

@@ -0,0 +1,13 @@
+// test for null-pointer exception fix in fast adaptive uniformisation (cumulative reward operator)
+
+ctmc
+
+module m1
+  s: [0..1] init 0;
+
+  [] s=0 -> 2:(s'=1);
+endmodule
+
+rewards "steps"
+  [] true : 1;
+endrewards

prism-tests/bugfixes/ctmc-fau-npe.sm.props

@@ -0,0 +1,8 @@
+const double T;
+
+// RESULT (T=1): 0.8646645498740888
+// RESULT (T=2): 0.9816832980524708
+// RESULT (T=3): 0.9975207981483099
+// RESULT (T=4): 0.9996641287431798
+// RESULT (T=5): 0.9999538916843567
+R=? [ C<=T ];

prism-tests/bugfixes/ctmc-fau-npe.sm.props.args

@@ -0,0 +1,3 @@
+-const T=1:1:5 -transientmethod unif
+-const T=1:1:5 -transientmethod fau
+

prism/src/explicit/FastAdaptiveUniformisation.java

@@ -1271,11 +1271,11 @@ public final class FastAdaptiveUniformisation extends PrismComponent
 						for (int j = 0; j < numChoices; j++) {
 							int numTransitions = modelGen.getNumTransitions(j);
 							for (int k = 0; k < numTransitions; k++) {
-								String tAction = modelGen.getTransitionAction(j, k).toString();
+								Object tAction = modelGen.getTransitionAction(j, k);
 								if (tAction == null) {
 									tAction = "";
 								}
-								if (tAction.equals(action)) {
+								if (tAction.toString().equals(action)) {
 									sumReward += reward * modelGen.getTransitionProbability(j, k);
 								}
 							}