[FIX] read() method returns None values instead of False, incompatible with XML-RPC

This error only happens with XML-RPC (not JSON-RPC).
After creating a database, calling `read()` on `res.users` or `res.partner` (others models might be concerned) raise this error on the server side:

```python
Traceback (most recent call last):
  File "/home/odoo/odoo/server/openerp/service/wsgi_server.py", line 76, in xmlrpc_return
    response = xmlrpclib.dumps((result,), methodresponse=1, allow_none=False, encoding=None)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1085, in dumps
    data = m.dumps(params)
  File "/usr/lib/python2.7/xmlrpclib.py", line 632, in dumps
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 714, in dump_array
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 735, in dump_struct
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 658, in dump_nil
    raise TypeError, "cannot marshal None unless allow_none is enabled"
TypeError: cannot marshal None unless allow_none is enabled
```
Fields returning `None` values were `image`, `image_small` and `image_medium` (all fields.Binary).

I thought the right place to fix this issue was in the `convert_to_read()` method of the `openerp.fields.Field` class, but I may be wrong.