Add handling to pipeline environment substitution for empty strings

[charlesbluca]

Description

Follow-up to #1457, modifying the main pull request workflow so that libmode tests are no longer getting skipped on all PRs against main

In doing the above, found that it's possible to error at the pipeline loading stage if relevant environment variables are set to an empty string (""); an example of these failures in our libmode tests:

https://github.com/NVIDIA/NeMo-Retriever/actions/runs/22584850034/job/65426873204

E           ValueError: Failed to parse YAML after environment variable substitution. Error: while parsing a flow node
E           expected the node content, but found ','
E             in "<unicode string>", line 55, column 11:
E                         ,
E                         ^
E           
E           --- Substituted Content ---
...
E                   yolox_endpoints: [
E                     "page-elements:8001",
E                     ,
E                   ]
...

Repurposing this PR to add handling to our YAML pipeline substitution logic for this case

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• If adjusting docker-compose.yaml environment variables have you ensured those are mimicked in the Helm values.yaml file.