property_sheets: set default value on SimpleItem.short_title

We have DublinCore.short_title and SimpleItem.short_title, make the two
use the same default value for consistency.

product/ERP5/bootstrap/erp5_property_sheets/PropertySheetTemplateItem/portal_property_sheets/SimpleItem/short_title_property.xml

@@ -49,6 +49,10 @@
             <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>
         <item>
             <key> <string>translatable</string> </key>
             <value> <int>1</int> </value>