Fix null pointer exception in TestNG when invalid options are used

Using invalid options through `cucumber.option` would cause the
creation of `testNGCucumberRunner` to fail with an exception. However
this exception does not terminate TestNGs execution. This caused
additional exceptions in the data provider and tear down methods. The
additional exceptions were mistaken for the root cause, creating some
confusion.

Author: mpkorstanje

testng/src/main/java/cucumber/api/testng/AbstractTestNGCucumberTests.java

@@ -29,11 +29,17 @@ public void runScenario(PickleEventWrapper pickleWrapper, CucumberFeatureWrapper
      */
     @DataProvider
     public Object[][] scenarios() {
+        if (testNGCucumberRunner == null) {
+            return new Object[0][0];
+        }
         return testNGCucumberRunner.provideScenarios();
     }
 
     @AfterClass(alwaysRun = true)
     public void tearDownClass() throws Exception {
+        if (testNGCucumberRunner == null) {
+            return;
+        }
         testNGCucumberRunner.finish();
     }
 }

testng/src/test/java/cucumber/api/testng/AbstractTestNGCucumberTestsInvalidArgument.java

@@ -0,0 +1,45 @@
+package cucumber.api.testng;
+
+import cucumber.runtime.CucumberException;
+import org.testng.Assert.ThrowingRunnable;
+import org.testng.annotations.AfterTest;
+import org.testng.annotations.BeforeTest;
+import org.testng.annotations.Test;
+
+import static java.util.Arrays.deepEquals;
+import static org.testng.Assert.assertThrows;
+import static org.testng.Assert.assertTrue;
+
+@Test
+public final class AbstractTestNGCucumberTestsInvalidArgument {
+
+    private final AbstractTestNGCucumberTests testCase = new AbstractTestNGCucumberTests() {
+
+    };
+
+    @BeforeTest(alwaysRun = true)
+    public void setup() {
+        System.setProperty("cucumber.options", "--invalid-option is invalid");
+        assertThrows(CucumberException.class, new ThrowingRunnable() {
+            @Override
+            public void run() throws Throwable {
+                testCase.setUpClass();
+            }
+        });
+    }
+
+    @AfterTest(alwaysRun = true)
+    public void tearDown() {
+        System.setProperty("cucumber.options", "");
+    }
+
+    @Test
+    public void dataProviderShouldProvideEmptyArrayWhenInvalidArgumentsAreUsed() {
+        assertTrue(deepEquals(new Object[0][0], testCase.scenarios()));
+    }
+
+    @Test
+    public void tearDownShouldNotThrowWhenInvalidArgumentsAreUsed() throws Exception {
+        testCase.tearDownClass();
+    }
+}