don't display maven version selector if there's only one option to choose from.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@4949 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/resources/hudson/tasks/Maven/config.jelly

 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
-  <f:entry title="Maven Version">
-    <select class="setting-input" name="maven_version">
-      <option>(Default)</option>
-      <j:forEach var="inst" items="${descriptor.installations}">
-        <f:option selected="${inst.name==instance.maven.name}">${inst.name}</f:option>
-      </j:forEach>
-    </select>
-  </f:entry>
+  <j:if test="${!empty(descriptor.installations)}">
+    <f:entry title="Maven Version">
+      <select class="setting-input" name="maven_version">
+        <option>(Default)</option>
+        <j:forEach var="inst" items="${descriptor.installations}">
+          <f:option selected="${inst.name==instance.maven.name}">${inst.name}</f:option>
+        </j:forEach>
+      </select>
+    </f:entry>
+  </j:if>
   <f:entry title="Goals">
     <f:expandableTextbox name="maven_targets" value="${instance.targets}" />
   </f:entry>