pkg/rpcserver: tolerate the kernel directory absence

[a-nogikh]

For fuzzing, we don't strictly need the kernel directory or the kernel object file. We just need a disk/kernel image.

[dvyukov]

We provide kernel_obj during syzbot builds, right? And it's generally assumed to be provided.
I think it's better if the test will be more realistic and create real config, rather than have a non-realistic test and add more production code to workaround that.

[a-nogikh]

The problem is not in the presubmit test, the problem is that syzkaller has just stopped working without kernel_obj (de facto without the kernel build directory), while it did not require it before.

[dvyukov]

Okay, but I am surprised we observe the difference anywhere. All our configs have all dirs filled.