fix: handle missing external_providers_dir (#3974)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
This PR fixes the handling of the external_providers_dir configuration
field to align with its ongoing deprecation, in favor of the provider
`module` specification approach.

It addresses the issue in #3950, where using the default provided
run.yaml config resulted in the `external_providers_dir` parameter being
set to the literal string `None`, and crashing the llama-stack server
when starting.

<!-- If resolving an issue, uncomment and update the line below -->
Closes #3950 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

- Built a new container image from `podman build . -f
containers/Containerfile --build-arg DISTRO_NAME=starter --tag
llama-stack:starter`
- Tested it locally with `podman run -it localhost/llama-stack:starter`
- Tested it on an OpenShift 4.19 cluster, deployed via the
llama-stack-k8s-operator.

Signed-off-by: Doug Edgar <dedgar@redhat.com>

llama_stack/cli/stack/run.py

@@ -106,7 +106,8 @@ class StackRun(Subcommand):
 
             try:
                 config = parse_and_maybe_upgrade_config(config_dict)
-                if not os.path.exists(str(config.external_providers_dir)):
+                # Create external_providers_dir if it's specified and doesn't exist
+                if config.external_providers_dir and not os.path.exists(str(config.external_providers_dir)):
                     os.makedirs(str(config.external_providers_dir), exist_ok=True)
             except AttributeError as e:
                 self.parser.error(f"failed to parse config file '{config_file}':\n {e}")