[Upgrade to SF2.3.18] Prevent serialization of job objects

| Q                    | A
| -------------------- | ---
| Bug fix?             |
| New feature?         |
| BC breaks?           |
| CI currently passes? |
| Tests pass?          |
| Scenarios pass?      |
| Checkstyle issues?   |
| Changelog updated?   |
| Fixed tickets        |
| Doc PR               |

https://github.com/symfony/symfony/commit/73d56f7a1a7e2aaf7cb00230af25ca208330ce6d

Job object serialization fails for unexpected reason (serialize() must
return NULL or string), I suspect one of the object descendant's
serialize method to be badly implemented. Sadly this is hardly
debuggable as the serialization tree is quite huge (more than 200 levels
of recursivity).