Add defaultValue to file-widget default config

[jacobtylerwalls]

Types of changes

• Bugfix (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)

Description of Change

Previously, saving an empty tile through the file widget raised TypeError, as None was saved, not an empty list.

Issues Solved

Closes #10176

Checklist

• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Testing instructions

• you can save an empty tile in file widget
• file widget doesn't start from dirty state (with "cancel edit")
• regression test saving and deleting files

[chiatt]

This works great for new nodes, but existing nodes with records in the cards_x_nodes_x_widgets table won't have the default value. I think we'd just have to apply similar sql to that cards_x_nodes_x_widgets table's config column (where widgetid = '10000000-0000-0000-0000-000000000019'). You can create a record in cards_x_nodes_x_widgets by giving your widget a custom label.

[jacobtylerwalls]

Great spot. I made those changes, but found they had no effect unless the graph was republished. Added programmatic graph republishing to the migration, LMK what you think.

[chiatt]

Works like a charm. Nice catch with the graph publication.