--dump-initial-registers-yaml shall work without model plugin

Reproduce on 1.0 release.

Snippy line:

```
./llvm-snippy -march=riscv64-linux-gnu -num-instrs=1000 -seed=0 -model-plugin=None ./yml/layout.yaml --init-regs-in-elf
 --dump-initial-registers-yaml
```

You will not see register dump because no model supplied. Still there will be some initial state provided by init-regs-in-elf option. We definitely can dump it.