if you set 0 as the maximum or the minimum allowed value on a Float or Integer field definition, the validatorConfiguration will contain false instead of 0

Steps to reproduce:
1. create a content type with a float or integer field definition with 0 as the max or min value
2. fetch the content type with REST for instance with: curl -u admin:ezpublish 'http://ezpublish5.loc/api/ezp/v2/content/types/XX' -H 'Accept: application/vnd.ez.api.ContentType+json'

the validatorConfiguration contains false instead of 0

PR and comments: https://github.com/ezsystems/ezpublish-kernel/pull/773