Fix 500 when creating text widgets without visualization_id

[amirshahzadhashmi7145]

Summary

• POST /api/widgets no longer 500s when visualization_id is omitted (text widgets).
• Fixes WidgetListResource.post() raises unhandled KeyError (500) when visualization_id is omitted for text widgets #7793

Test plan

• POST /api/widgets with {dashboard_id, width, text} (no visualization_id) returns 200
• Same request with "visualization_id": null still returns 200
• Creating a visualization widget still works

[greptile-apps]

Greptile Summary

The PR prevents text-widget creation from failing when visualization_id is omitted.

• Defaults an absent visualization_id to None, matching the existing explicit-null behavior.
• Adds handler coverage for creating a text widget without the key.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
redash/handlers/widgets.py	Safely treats an omitted visualization identifier like the already-supported explicit-null case.
tests/handlers/test_widgets.py	Adds focused regression coverage for text-widget creation without visualization_id.

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[cubic-dev-ai]

No issues found across 2 files

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