fix: Read ASR and DTMF if not included

[Ruari-Phipps]

Summary

If ASR and DTMF not included, still read them as defaults

Motivation

Would error if created without these fields

Changes

• Filter on None not empty dicts

Test strategy

• Added/updated unit tests
• Manual CLI testing (poly <command>)
• Tested against a live Agent Studio project
• N/A (docs, config, or trivial change)

Checklist

• ruff check . and ruff format --check . pass
• pytest passes
• No breaking changes to the poly CLI interface (or migration path documented)
• Commit messages follow conventional commits

Screenshots / Logs