Update secrets check to allow empty placehodlers

[diondrapeck]

Description

This PR updates the secret detection pre-commit hook script to ignore environment variable patterns for azure secrets if the variable is set to an empty strings. This allows sample creators to include placeholders for secrets in their samples.

I've also updated the contributing guide to recommend the empty string syntax for these placeholders.

Checklist

• I have read the contribution guidelines
• I have coordinated with the docs team ([email protected]) if this PR deletes files or changes any file names or file extensions.
• This notebook or file is added to the CODEOWNERS file, pointing to the author or the author's team.