[BUG] bugfix when cluster has no workers and key error happens when fetching num cores

[samster25]

error in question

virtualenv/lib/python3.8/site-packages/daft/runners/ray_runner.py", line 529, in _run_plan
    cores: int = next(num_cpus_provider) - self.reserved_cores
  File "/tmp/ray/session_2023-12-14_10-43-11_589430_8/runtime_resources/pip/6268584d5ab6509fe9044d782b6939e786ec5677/virtualenv/lib/python3.8/site-packages/daft/runners/ray_runner.py", line 400, in _ray_num_cpus_provider
    last_num_cpus_queried = int(ray.cluster_resources()["CPU"])
KeyError: 'CPU'

[codecov]

Codecov Report

Merging #1745 (a64b40c) into main (e4e8528) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1745      +/-   ##
==========================================
- Coverage   85.19%   85.03%   -0.17%     
==========================================
  Files          55       55              
  Lines        5518     5518              
==========================================
- Hits         4701     4692       -9     
- Misses        817      826       +9     

Files	Coverage Δ
daft/runners/ray_runner.py	87.79% <100.00%> (-1.18%)	⬇️

... and 2 files with indirect coverage changes