Exception when reseting password if first field is empty

Related: #264 and #510 

```
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/controllers/user.py', line 447 in perform_reset
  user = get_action('user_update')(context, user_dict)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/logic/__init__.py', line 323 in wrapped
  return _action(context, data_dict, **kw)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/logic/action/update.py', line 586 in user_update
  data, errors = _validate(data_dict, schema, context)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/lib/navl/dictization_functions.py', line 228 in validate
  converted_data, errors = _validate(flattened, schema, context)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/lib/navl/dictization_functions.py', line 290 in _validate
  convert(converter, key, converted_data, errors, context)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/lib/navl/dictization_functions.py', line 186 in convert
  converter(key, converted_data, errors, context)
File '/home/adria/dev/pyenvs/ckan_plain/src/ckan/ckan/logic/validators.py', line 467 in user_password_validator
  if not value == '' and not isinstance(value, Missing) and not len(value) >= 4:
TypeError: object of type 'NoneType' has no len()
```