fix: resolve HuggingFace Spaces runtime error during deployment

[u-sheikh]

The issue was in the username retrieval logic in start.sh. When the HuggingFace API returns an error (e.g., user not found), jq would return the literal string "null" instead of triggering the fallback to SPACE_AUTHOR_NAME.

Changes:

• Use jq's '// empty' alternative operator to return empty string instead of "null" when the .user field is missing or null
• Use bash parameter expansion to properly fallback to SPACE_AUTHOR_NAME
• Add proper quoting around the curl URL

This fixes the runtime error reported in issue #5845.

Description

Closes #<issue_number>

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)