Fix NPE during tests caused by null exception message

Author: ljleppan

tmc-langs-java/src/main/java/fi/helsinki/cs/tmc/langs/java/TestResultParser.java

@@ -77,7 +77,9 @@ private TestResult convertTestCaseResult(fi.helsinki.cs.tmc.testrunner.TestCase
         List<String> points = new ArrayList<>();
 
         if (testCase.exception != null) {
-            exception.add(testCase.exception.message);
+            if (testCase.exception.message != null) {
+                exception.add(testCase.exception.message);
+            }
             for (StackTraceElement stackTrace : testCase.exception.stackTrace) {
                 exception.add(stackTrace.toString());
             }

tmc-langs-java/src/test/java/fi/helsinki/cs/tmc/langs/java/maven/MavenPluginTest.java

@@ -189,7 +189,7 @@ public void testMultipleTestMethodsGetAddedToRunResultWhenSomeFail() {
 
         assertTrue(result.testResults.get(0).getException().isEmpty());
         assertFalse(
-                "Lol:" + result.testResults.get(1).getException(),
+                "Expected not empty, was:" + result.testResults.get(1).getException(),
                 result.testResults.get(1).getException().isEmpty());
     }