core: fix error when submitting "View Diff" dialog

This action was working fine the first time displayed, but clicking
again on "View Diff" button was causing an IndexError: list index out of range
when used on one document.

This was because in this case list_selection_name exists in REQUEST, but
is an empty string. Instead of checking that this selection name is not
None to detect if we are in module mode or in document mode, check that
its true-ish to also tolerate an empty string.

bt5/erp5_ui_test/PathTemplateItem/portal_tests/page_templates_zuite/testDiffActionOnModule.zpt

@@ -63,48 +63,56 @@
   <td>Diff Module Objects</td>
 </tr>
 
-<!-- Check the diff content -->
-<tr>
+<tal:block tal:repeat="_ python: (1, 2)">
+
+  <!-- Check the diff content -->
+  <tr>
     <td>verifyValue</td>
     <td>field_your_first_path</td>
     <td>bar_module/1</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>verifyValue</td>
     <td>field_your_second_path</td>
     <td>bar_module/2</td>
-</tr>
+  </tr>
 
-<tr>
+  <tr>
     <td>assertText</td>
     <td>//a[@class="listbox_title"]</td>
     <td>exact:Diff</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>waitForElementPresent</td>
     <td>//tr[@class='listbox-data-line-2 DataA']</td>
     <td></td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>assertText</td>
     <td>//tr[@class='listbox-data-line-2 DataA']/td[1]</td>
     <td>title</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>assertElementPresent</td>
     <td>//tr[@class='listbox-data-line-2 DataA']/td[2]/div[@data-gadget-sandbox="public"]</td>
     <td></td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>storeEval</td>
     <td>document.querySelector('#selenium_myiframe').contentWindow.document.evaluate("//tr[@class='listbox-data-line-2 DataA']/td[2]/div[@data-gadget-sandbox='public']", document.querySelector('#selenium_myiframe').contentWindow.document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue.getAttribute('data-gadget-value');</td>
     <td>_data_gadget_value</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>verifyEval</td>
     <td>storedVars['_data_gadget_value'].replace(/(\r\n\t|\n|\r\t)/gm,"").replace(/\s+/g, '')</td>
     <td>---+++@@-1+1@@-Title1+Title2</td>
-</tr>
-
+  </tr>
+  <!-- submit the action again, it should be same -->
+  <tr>
+    <td>clickAndWait</td>
+    <td>dialog_submit_button</td>
+    <td></td>
+  </tr>
+</tal:block>
 </body>
 </html>
\ No newline at end of file

bt5/erp5_ui_test/PathTemplateItem/portal_tests/page_templates_zuite/testDiffActionOnObject.zpt

@@ -54,48 +54,58 @@
   <td>Diff Object</td>
 </tr>
 
-<!-- Check the diff content -->
-<tr>
+<tal:block tal:repeat="_ python: (1, 2)">
+
+  <!-- Check the diff content -->
+  <tr>
     <td>verifyValue</td>
     <td>field_your_first_path</td>
     <td>bar_module/1</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>verifyValue</td>
     <td>field_your_second_path</td>
     <td>bar_module/1</td>
-</tr>
+  </tr>
 
-<tr>
+  <tr>
     <td>assertText</td>
     <td>//a[@class="listbox_title"]</td>
     <td>exact:Diff</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>waitForElementPresent</td>
     <td>//tr[@class='listbox-data-line-1 DataB']</td>
     <td></td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>assertText</td>
     <td>//tr[@class='listbox-data-line-1 DataB']/td[1]</td>
     <td>title</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>assertElementPresent</td>
     <td>//tr[@class='listbox-data-line-1 DataB']/td[2]/div[@data-gadget-sandbox="public"]</td>
     <td></td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>storeEval</td>
     <td>document.querySelector('#selenium_myiframe').contentWindow.document.evaluate("//tr[@class='listbox-data-line-1 DataB']/td[2]/div[@data-gadget-sandbox='public']", document.querySelector('#selenium_myiframe').contentWindow.document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue.getAttribute('data-gadget-value');</td>
     <td>_data_gadget_value</td>
-</tr>
-<tr>
+  </tr>
+  <tr>
     <td>verifyEval</td>
     <td>storedVars['_data_gadget_value'].replace(/(\r\n\t|\n|\r\t)/gm,"").replace(/\s+/g, '')</td>
     <td>---+++@@-1+1@@-Title1+Title2</td>
-</tr>
+  </tr>
+
+  <!-- submit the action again, it should be same -->
+  <tr>
+    <td>clickAndWait</td>
+    <td>dialog_submit_button</td>
+    <td></td>
+  </tr>
 
+</tal:block>
 </body>
 </html>
\ No newline at end of file

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_diff/Base_getObjectDiffList.py

@@ -98,7 +98,7 @@ if ((first_path is None and second_path is None) and
   # In case the list_selection_name is there, it can be the case of selection
   # from the module, hence we get the paths from the selection and use them to
   # create diff.
-  if list_selection_name is not None:
+  if list_selection_name:
     selected_object_list = portal.portal_selections.getSelectionCheckedValueList(
       selection_name=list_selection_name)
     first_object = selected_object_list[0]