DataForm: Fix panel field inaccessible when empty with labelPosition none

[jorgefilipecosta]

This PR Adds a fallback placeholder for empty panel fields when labelPosition is set to none.

Why?

When a DataForm panel field has labelPosition: none and the field value is empty, the summary button renders with no content, collapsing to zero width. This makes the field inaccessible since there's nothing to click to open the panel. There is no way a use can open the field panel and make the field not empty.

How?

Display "(Empty field)" text when labelPosition === 'none' and all summary field values are empty.

Testing

1. Open Storybook DataForm panel layout story with labelPosition: none http://localhost:50240/?path=/story/dataviews-dataform--layout-panel&args=labelPosition:none
2. Clear the Title field "(Hello World)".
3. Verify the placeholder appears and clicking it opens the panel

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Co-authored-by: jorgefilipecosta <[email protected]>
Co-authored-by: ntsekouras <[email protected]>
Co-authored-by: jameskoster <[email protected]>

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[jorgefilipecosta]

cc: @oandregal, @jameskoster

[ntsekouras]

I'm still familiarizing myself with DF, but I think:

1. the same problem occurs even with a label
2. the same problem occurs at least in card layout

When empty should we use for summaryContent an edit: ${attribute label} instead of empty field? I think that's better, but if we went for an empty field approach, it should be different based on the number of fields.

Finally with this change in the story ?path=/story/dataviews-dataform--layout-panel&args=labelPosition:none the discussion appears empty. Is this in indicator that we need a getValue there in general?

[jameskoster]

edit: ${attribute label}

I had a similar thought 👍

Edit: I realised this is a bit related to #73036. "${attribute label} unset" could also be an option for when the label is hidden. When the label is visible we might not need to repeat it.