Scheduler terminates if node was not initialized

After fix #24 (closed), there is another exception:

Traceback (most recent call last):
  File "/home/gitlab-runner/builds/Ei-uk5TxX/0/verification/klever/venv/bin/klever-native-scheduler", line 8, in <module>
    sys.exit(native_scheduler())
  File "/home/gitlab-runner/builds/Ei-uk5TxX/0/verification/klever/venv/lib/python3.10/site-packages/klever/scheduler/main.py", line 56, in native_scheduler
    scheduler_impl = Scheduler(conf, logger, "scheduler/", native.Native)
  File "/home/gitlab-runner/builds/Ei-uk5TxX/0/verification/klever/venv/lib/python3.10/site-packages/klever/scheduler/schedulers/__init__.py", line 111, in __init__
    self.init_scheduler()
  File "/home/gitlab-runner/builds/Ei-uk5TxX/0/verification/klever/venv/lib/python3.10/site-packages/klever/scheduler/schedulers/__init__.py", line 135, in init_scheduler
    self.runner.init()
  File "/home/gitlab-runner/builds/Ei-uk5TxX/0/verification/klever/venv/lib/python3.10/site-packages/klever/scheduler/schedulers/native.py", line 98, in init
    raise ValueError(f'Expect strictly single active connected node but {len(nodes)} given')
ValueError: Expect strictly single active connected node but 0 given