[FrameworkBundle] skip compiler pass if interface doesn't exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12813
| License       | MIT
| Doc PR        | 

The FrameworkBundle doesn't depend on the `symfony/translation`
component and therefore does not enforce a high enough version of this
package. Thus, with `symfony/translation` versions before `2.6`, the
`translator` service does exist, but the `TranslatorBagInterface` is
not present.