Sets _format attribute only if it wasn't set previously by the user.

[HttpKernel] [Fragment] [InlineFragmentRenderer] Fixes #8787

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8787 
| License       | MIT
| Doc PR        |