[HttpKernel] fix overwriting of request's locale by LocaleListener if attribute _locale is missing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7474
| License       | MIT
| Doc PR        | n.a.