fix startup when no plugins are configured

[cebtenzzre]

I ran into this while testing a minimal configuration of AppDaemon. It seems to be a regression caused by commit d255a5e, part of the 4.5.12 release.

Fixes this error:

ERROR    asyncio:base_events.py:1879 Task exception was never retrieved
future: <Task finished name='waiting for all plugins to be ready' coro=<wait() done, defined at /home/jared/.local/share/uv/python/cpython-3.13.9-linux-x86_64-gnu/lib/python3.13/asyncio/tasks.py:424> exception=ValueError('Set of Tasks/Futures is empty.')>
Traceback (most recent call last):
  File "/home/jared/.local/share/uv/python/cpython-3.13.9-linux-x86_64-gnu/lib/python3.13/asyncio/tasks.py", line 441, in wait
    raise ValueError('Set of Tasks/Futures is empty.')
ValueError: Set of Tasks/Futures is empty.