The contract is that libX_foo can return null. But a specific implementation might not. Now you need to remove the caller side check to shut up the compiler which will leave you exposed to a future update making full use of the contract.

Also consider code that call libX_foo via a pointer. After specialization the compiler might see that the check is redundant, but you can't remove the check because the function might still be called with other function pointers making full use of the contract.