fix: PromptNode run raises on empty inputs

[tstadel]

Related Issues

• fixes raising the following error if running prompt_node.run(query="", documents=documents)

  Exception while running node 'PromptNode': Expected prompt parameters ['documents', 'query'] to be provided but got only 
  ['documents']. Make sure to provide all template parameters.
  Enable debug logging to see the data that was passed when the pipeline failed.
  

Proposed Changes:

• include any variable into invocation_context that is not None

How did you test it?

• added test

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue