[deliver] Fixed crash on running 'deliver download_metadata'

[ha1f]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

If we run fastlane deliver download_metadata with empty content, it crashes and the process will be interrupted.

closes #16949

Description

I use default string for nil.

Testing Steps

[joshdholtz]

Love it! Thanks so much for making this fix ❤️ I really appreciate the contribution!

[SebaRossi94]

Hello there, should i update fastlane in order to be able to run fastlane deliver init?
Thanks for the quick fix

[fastlane-bot]

Hey @ha1f 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.154.0 🚀