OOoParser: Handle empty cells properly

This is a fix for 841b0206 which started to
return "" for empty cells, whereas it's supposed to return None.
This also fixes the 20 minutes slowdown introduced when parsing a category
spreadsheet, such as in the first page of configurator.

product/ERP5OOo/OOoUtils.py

@@ -464,7 +464,7 @@ class OOoParser(Implicit):
                   return ''.join(part for part in
                     [node.text, node.tail] if part)
                 # we can also have table:annotation, and they are ignored
-              cell_data = format_node(cell)
+              cell_data = format_node(cell) or None
 
             # Add the cell to the line
             table_line.append(cell_data)

product/ERP5OOo/tests/testOOoParser.py

@@ -116,6 +116,18 @@ class TestOOoParser(unittest.TestCase):
     self.assertEquals(mapping['Feuille1'][2], ['tab\t'])
     self.assertEquals(mapping['Feuille1'][3], ['New\nLine'])
 
+  def test_getSpreadSheetMappingEmptyCells(self):
+    parser = OOoParser()
+    parser.openFile(open(makeFilePath('empty_cells.ods'), 'rb'))
+    mapping = parser.getSpreadsheetsMapping()
+    self.assertEquals(['Feuille1'], mapping.keys())
+    self.assertEquals(mapping['Feuille1'],
+      [
+        ['A1', None, 'C1'],
+        [],
+        [None, 'B3',],
+      ])
+
 
 def test_suite():
   suite = unittest.TestSuite()