set default value to '', because if a value is None, render_element will display value="value"

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@28644 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/Formulator/Widget.py

@@ -1280,7 +1280,7 @@ class FloatWidget(TextWidget):
                                       title='Precision',
                                       description=(
       "Number of digits after the decimal point"),
-                                      default=None,
+                                      default='',
                                       required=0)
 
   def format_value(self, field, value):