fix NPE

src/main/java/de/tlc4b/Log.java

@@ -58,9 +58,11 @@ public class Log {
 		fieldNames.add("Operation Coverage");
 		Map<String, Long> operationCount = tlcResults.getOperationCount();
 		List<String> opCountString = new ArrayList<>();
+		if (operationCount != null) {
 			for (String operation : operationCount.keySet()) {
 				opCountString.add(operation + DELIMITER + operationCount.get(operation));
 			}
+		}
 		fieldValues.add(!opCountString.isEmpty() ? String.join(DELIMITER, opCountString) : "");
 
 		fieldNames.add("Trace File");