Fixes symfony serializer being mandatory

[magnetik]

Fixes #1937

I've tested locally but maybe we should have a functional test with symfony serializer not installed?

[GuilhemN]

Thanks!

I've tested locally but maybe we should have a functional test with symfony serializer not installed?

I guess the only way to test that would be through github actions, by adding more cases to the matrix, this would need some rethinking of our tests though to skip all tests were the Symfony serializer is required...