Fix prepare_lookup_value to handle '0' for _isnull

In contrib.admin.util, there is a prepare_lookup_value function

In this function, there is a clause

    if value.lower() in ('', 'false'):
            value = False

I think it should be

    if value.lower() in ('', 'false', '0'):
          value = False

Note that in admin already converts a limit_choices_to={'a__isnull': False} to a__isnull=0 when formatting a query string for lookup on the remote model. Therefore, clearly it should also handle isnull=0 correctly. Currently the lookup popup box on remote model uses __isnull=0 and does not actually filter.