[FIX] web, orm: allow 0 as key for selection fields

Before this revision,
using a selection field with as possible keys `0`,
such as `require_payment` in `website_quote`,
resulted to the records field values to be stored
as `NULL` instead of `0` in the database.

This leads to the inability:
 - To distinguish records with this selection field not set
   or set to `0` (as both are stored as `NULL`)
 - To search for records with the field set to `0`
   (as they are considered not set)

For instance, while having `website_quote` installed,
search for sales orders with `Payment` set to
`Not mandatory on website quote validation`: It won't
return any result, even if you have some.

opw-697454

addons/web/static/src/js/views/form_widgets.js

@@ -955,7 +955,8 @@ var FieldRadio = common.AbstractField.extend(common.ReinitializeFieldMixin, {
     },
     get_value: function () {
         var value = this.get('value');
-        return ((value instanceof Array)? value[0] : value) || false;
+        value = ((value instanceof Array)? value[0] : value);
+        return  _.isUndefined(value) ? false : value;
     },
     render_value: function () {
         var self = this;

odoo/fields.py

@@ -1744,6 +1744,14 @@ class Selection(Field):
                 return item[1]
         return False
 
+    def convert_to_column(self, value, record):
+        """ Convert ``value`` from the ``write`` format to the SQL format. """
+        if value is None or value is False:
+            return None
+        if isinstance(value, unicode):
+            return value.encode('utf8')
+        return str(value)
+
 
 class Reference(Selection):
     type = 'reference'