WW-5063 Adds tests to cover new functionality

plugins/json/src/test/java/org/apache/struts2/json/JSONResultTest.java

@@ -35,6 +35,7 @@ import java.util.regex.Pattern;
 
 import javax.servlet.http.HttpServletResponse;
 
+import com.opensymphony.xwork2.Result;
 import org.apache.struts2.StrutsStatics;
 import org.apache.struts2.StrutsTestCase;
 import org.apache.struts2.util.TestUtils;
@@ -713,6 +714,16 @@ public class JSONResultTest extends StrutsTestCase {
         assertEquals("UTF-8", encoding);
     }
 
+    public void testPassingNullInvocation() throws Exception{
+        Result result = new JSONResult();
+        try {
+            result.execute(null);
+            fail("Exception should be thrown!");
+        } catch (IllegalArgumentException e) {
+            assertEquals("Invocation cannot be null!", e.getMessage());
+        }
+    }
+
     @Override
     protected void setUp() throws Exception {
         super.setUp();