fix: controlled input warning (#1654)

Fixes the SourceForm `name` input switching from uncontrolled to controlled.

<img width="753" height="272" alt="image" src="https://github.com/user-attachments/assets/818a7489-d237-416b-8826-a584ddf49fe5" />

Author: karl-power

packages/app/src/components/Sources/SourceForm.tsx

@@ -1611,7 +1611,7 @@ export function TableSourceForm({
   onSave,
   onCreate,
   isNew = false,
-  defaultName,
+  defaultName = '',
   onCancel,
 }: {
   sourceId?: string;