fix default comparison in AbstractFilter, cause Propel ORM need NUL...

...L and don't accept empty string

because '' (empty string) fallback into this class and can't be parsed like Criteria::EQUAL
see propel/propel1/runtime/lib/query/Criterion.php

Of course, tests was passed

```
> 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: 1.07 minutes, Memory: 73.00Mb

OK (185 tests, 474 assertions)

Process finished with exit code 0 at 13:40:16.
Execution time: 64 355 ms.