🛠️ Description

Verify billing account ID parameter to avoid null reference error.

Fixes #473

📋 Checklist

🔬 How did you test this change?

• 🤞 PS -WhatIf / az validate
• 👍 Manually deployed + verified
• 💪 Unit tests

🙋‍♀️ Do any of the following that apply?

• 🚨 This is a breaking change.
• 🤏 The change is less than 20 lines of code.

📑 Did you update docs/changelog.md?

• ✅ Yes (required for dev PRs)
• ➡️ Will cover in a future PR (feature branch PRs only)
• ❎ Not needed (small/internal change)

📖 Did you update documentation?

• ✅ Public docs in docs (required for dev)
• ✅ Internal dev docs in src (required for dev)
• ➡️ Will cover in a future PR (feature branch PRs only)
• ❎ Not needed (small/internal change)