iss1007: fix NPE in SPIMComputationAccessor

haas-spim-benchmark/src/main/java/cz/it4i/fiji/haas_spim_benchmark/core/SPIMComputationAccessorDecoratorWithTimeout.java

@@ -23,7 +23,7 @@ public class SPIMComputationAccessorDecoratorWithTimeout implements SPIMComputat
 				set.clear();
 			}
 			set.addAll(decorated.getChangedFiles());
-			return null;
+			return set;
 		});
 	}