Prevent serialization of job objects

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).