Fix notice about undefined index in AbstractFilter

All tests passed

```
hidem@hidem-zs:/opt/data/sites/PropelAdminBundle$ phpunit.phar
PHPUnit 3.7.29 by Sebastian Bergmann.

Configuration read from /opt/data/sites/PropelAdminBundle/phpunit.xml

...............................................................  63 / 185 ( 34%)
............................................................... 126 / 185 ( 68%)
...........................................................

Time: 1.05 minutes, Memory: 66.75Mb

OK (185 tests, 474 assertions)