[FrameworkBundle] Fixed validator factory definition when the Validator API is "auto" for PHP < 5.3.9

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11580
| License       | MIT
| Doc PR        | -
you should still keep the ``is_scalar`` check used previously. Otherwise, if someone puts an array in this config, he will get a warning because of the string casting before normalization instead of getting an exception because of invalid config