[erp5_property_sheets]: set default of text_content to empty string

Before, the text content of a new text document derived portal type
suche as Notebook was None. This lead to ui problems
(displaying None in text editor) and to download the data attachment
from jio because downloadable mixin would try to get len(None)

product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/TextDocument/text_content_property.xml

@@ -47,6 +47,10 @@
             <key> <string>portal_type</string> </key>
             <value> <string>Standard Property</string> </value>
         </item>
+        <item>
+            <key> <string>property_default</string> </key>
+            <value> <string>python: \'\'</string> </value>
+        </item>
       </dictionary>
     </pickle>
   </record>