Allow unsetting widget labels (#6773)

https://github.com/user-attachments/assets/af344318-dac2-4611-b080-910cdfa1e87d

Quick followup to #6752
- Adds support for placeholder values in dialogService.prompt
- When label is unset, initial prompt is empty
- Display original widget name as placeholder
- When prompt returns an empty string (as opposed to null for a canceled
operation), remove widget label

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6773-Allow-unsetting-widget-labels-2b16d73d365081ae9f5dd085d0081733)
by [Unito](https://www.unito.io)

src/services/litegraphService.ts

@@ -836,11 +836,12 @@ export const useLitegraphService = () => {
               const newLabel = await useDialogService().prompt({
                 title: t('g.rename'),
                 message: t('g.enterNewName') + ':',
-                defaultValue: overWidget.label ?? overWidget.name
+                defaultValue: overWidget.label,
+                placeholder: overWidget.name
               })
-              if (!newLabel) return
-              overWidget.label = newLabel
-              input.label = newLabel
+              if (newLabel === null) return
+              overWidget.label = newLabel || undefined
+              input.label = newLabel || undefined
               useCanvasStore().canvas?.setDirty(true)
             }
           })