Merge pull request #1750 from ikedam/feature/JENKINS-29064_DropdownDescriptorSelectorOptional

[JENKINS-29064] dropdownDescriptorSelector should accept descriptors without configurations.

core/src/main/resources/lib/form/dropdownDescriptorSelector.jelly

@@ -57,7 +57,7 @@ THE SOFTWARE.
         lazy="descriptor">
         <l:ajax>
           <j:set var="instance" value="${current.descriptor==descriptor ? current : null}" />
-          <st:include from="${descriptor}" page="${descriptor.configPage}" />
+          <st:include from="${descriptor}" page="${descriptor.configPage}" optional="true" />
         </l:ajax>
       </f:dropdownListBlock>
     </j:forEach>