Update DFP MLflow ModelManager to handle model retrieval using file U…

…RI (nv-morpheus#1578)

- Small update to handle MLflow `search_registered_models` results using file URI.

Closes nv-morpheus#1574

## By Submitting this PR I confirm:
- I am familiar with the [Contributing Guidelines](https://github.com/nv-morpheus/Morpheus/blob/main/docs/source/developer_guide/contributing.md).
- When the PR is ready for review, new or existing tests cover these changes.
- When the PR is ready for review, the documentation is up to date with these changes.

Authors:
  - Eli Fajardo (https://github.com/efajardo-nv)

Approvers:
  - Michael Demoret (https://github.com/mdemoret-nv)

URL: nv-morpheus#1578

examples/digital_fingerprinting/production/morpheus/dfp/utils/model_cache.py

@@ -225,7 +225,7 @@ def _model_exists(self, reg_model_name: str, timeout: float = 1.0) -> bool:
 
                         self._existing_models.update(model.name for model in results)
 
-                        if (len(results.token) == 0):
+                        if (results.token is None or len(results.token) == 0):
                             break
 
                     logger.debug("Updating list of available models... Done.")