Fix errors from empty 'resources' field in YAML.

[concretevitamin]

Repro: create a YAML like this

resources:
  # commented out!

Previously:

» sky launch test.yaml
...
File .. sky/backends/onprem_utils.py", line 492, in check_local_cloud_args
    yaml_cloud = yaml_config['resources'].get('cloud')
AttributeError: 'NoneType' object has no attribute 'get'

Now:

» sky launch test.yaml
Task from YAML spec: test.yaml
ValueError: Invalid task YAML: None is not of type 'object'. Check problematic field(s): $.resources

The fix is we do check_local_cloud_args() after Task ctor which performs schema validation.

Tested (run the relevant ones):

• Any manual or new tests for this PR (please specify below)
  • above

[romilbhardwaj]

Thanks @concretevitamin! Tried it out with a onprem cluster (and invalid onprem spec) to make sure onprem validation code path doesn't break - seems to work fine.