fix(frontend): allow fallback block to be empty

[omarNaifer12]

The backend does not allow fallback_block to be null it only allows it to be a string.

Fixes #553

Type of change:

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my own code
• New and existing unit tests pass locally with my changes

[MohamedAliBouhaouala]

Please rebase.

git checkout main
git pull upstream main
git checkout issue-553-fallback-block-can-be-empty
git rebase main