add null safety, more usable exception message

Author: sebtiem

maven-surefire-common/src/main/java/org/apache/maven/plugin/surefire/AbstractSurefireMojo.java

@@ -2882,9 +2882,10 @@ private List<CommandLineOption> commandLineOptions() {
 
     private void warnIfDefunctGroupsCombinations() throws MojoFailureException, MojoExecutionException {
         Artifact junitArtifact = getJunitArtifact();
-        if (!isWithinVersionSpec(junitArtifact, "[4.12,)")) {
-            throw new MojoFailureException(
-                    "The used JUnit Version is not supported anymore. Please update to version 4.12+");
+        if (junitArtifact != null && !isWithinVersionSpec(junitArtifact, "[4.12,)")) {
+            throw new MojoFailureException(String.format(
+                    "The used JUnit Version %s is not supported anymore. Please update to version 4.12+",
+                    junitArtifact.getVersion()));
         }
         if (isAnyGroupsSelected()) {
             if (getTestNgArtifact() == null) {