velociraptor: guard auth() against get_file() returning None

[chiefghost47]

Problem

Velociraptor.auth() reads the API credential with:

credfile = self.get_file(api_config)["data"]

When get_file() returns None — which happens when the worker cannot reach
the Shuffle files API (this is the case on distributed workers and in some
multi-org / suborg runtimes, where the runtime token only permits
/streams/results) — this line raises the opaque:

TypeError: 'NoneType' object is not subscriptable

Every velociraptor action then fails with that traceback and no indication of
the real cause, which makes it very hard to diagnose from the execution log.

Fix

Guard the get_file() result. If it returns usable data, use it as before.
Otherwise fall back to treating api_config as the credential contents
themselves (so the api.config.yaml can be supplied inline in the
authentication field), and raise a clear, actionable error if neither path
yields a credential:

_f = self.get_file(api_config)
credfile = _f["data"] if isinstance(_f, dict) and _f.get("data") else api_config
if not credfile:
    raise ValueError(
        "Velociraptor: could not load the API credential. get_file() "
        "returned no data - the worker may be unable to reach the "
        "Shuffle files API. Provide the api.config.yaml contents in the "
        "authentication field."
    )
config = yaml.load(credfile, Loader=yaml.FullLoader)

No behavior change for the normal case (files API reachable); it only adds a
fallback and a clear error for the environments where get_file() returns
None.

Testing

• Files API reachable: auth() behaves exactly as before.
• get_file() returns None with inline YAML in the auth field: credential
  loads from api_config and actions run.
• get_file() returns None with no usable credential: raises the descriptive
  ValueError instead of the NoneType TypeError.