[FIX] orm: do not save empty translation as 'false'

When writting a value on a translatable field in a different language than
English, the submitted *raw* value was saved in the database.

This could cause the following issues:
- empty value (provided as `False` by the web client) saved as the string
  'false' in the translations table
- no encoding or sanitization convertion
- ignore size parameter on the translatable field

Process the submitted translation through symbol_set method to clean it before
storing it blindly in the database.

This allows to convert `False` into `''` for empty value and fixes #10862

openerp/models.py

@@ -3910,7 +3910,8 @@ class BaseModel(object):
                             # Inserting value to DB
                             context_wo_lang = dict(context, lang=None)
                             self.write(cr, user, ids, {f: vals[f]}, context=context_wo_lang)
-                        self.pool.get('ir.translation')._set_ids(cr, user, self._name+','+f, 'model', context['lang'], ids, vals[f], src_trans)
+                        translation_value = self._columns[f]._symbol_set[1](vals[f])
+                        self.pool['ir.translation']._set_ids(cr, user, self._name+','+f, 'model', context['lang'], ids, translation_value, src_trans)
 
         # invalidate and mark new-style fields to recompute; do this before
         # setting other fields, because it can require the value of computed