Fixes for null checks and sorting issues

Sorting issues in the Inventory and Model Inventory Fields pages have been resolved. Previously, clicking column headers like Use Case or Key could cause the page to stop working. Now, all sortable columns function correctly. Non-sortable columns, such as In Workflows, are disabled to prevent confusion.

Additionally, an issue when editing models with empty Use Case fields has been fixed. Opening a model with an empty Use Case field no longer causes the page to stop working, as empty optional fields are now handled smoothly.