fix list when not found in stack and so is null

Also add tests for when user has supplied a not iterable list
See also WW-4910

core/src/main/java/org/apache/struts2/components/ListUIBean.java

@@ -87,7 +87,7 @@ public abstract class ListUIBean extends UIBean {
             value = list;
         }
 
-        if (value instanceof Iterable) {
+        if (value == null || value instanceof Iterable) {
             addParameter("list", value);
         } else if (MakeIterator.isIterable(value)) {
             addParameter("list", MakeIterator.convert(value));

core/src/test/java/org/apache/struts2/views/freemarker/FreemarkerResultMockedTest.java

@@ -236,9 +236,11 @@ public class FreemarkerResultMockedTest extends StrutsInternalTestCase {
         dispatcher.serviceAction(request, response, mapping);
         String result = stringWriter.toString();
         assertTrue(result.contains("<option value=\"a\">a</option>"));
+        assertTrue(result.contains("<option value=\"1\">1</option>"));
         assertTrue(result.contains("<option value=\"key\">value</option>"));
         assertTrue(result.contains("<option value=\"optgroupKey1\">optgroupValue1</option>"));
         assertTrue(result.contains("<option value=\"optgroupKey3\">optgroupKey3</option>"));
+        assertTrue(result.contains("<option value=\"2\">2</option>"));
     }
 
     private void init() throws MalformedURLException, URISyntaxException {

core/src/test/resources/org/apache/struts2/views/freemarker/select.ftl

@@ -19,8 +19,10 @@
  */
 -->
 <@s.select theme="simple" list=['a','b','c'] />
+<@s.select theme="simple" list=1 />
 <@s.select theme="simple" list={'key':'value'} />
 <@s.select theme="simple">
 <@s.optgroup label="label1" list={'optgroupKey1':'optgroupValue1','optgroupKey2':'optgroupValue2'} />
 <@s.optgroup label="label2" disabled=true list=['optgroupKey3','optgroupKey4'] />
+<@s.optgroup label="label3" disabled=true list=2 />
 </@s.select>