Add option to hide the table if the field has empty value

Problem/Motivation

When the field is empty, the widget displays a table without body rows, just a header with nothing below, and the "empty" message. This could be considered as non optimal UI.

Steps to reproduce

Open any entity form that uses this widget, and the field has empty value.

Proposed resolution

I propose a configuration setting that, if enabled, will hide the table and only show the button(s) and the "empty" message.

Remaining tasks

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User interface changes

When the field is empty, instead of a table with empty body there will just be a markup message.

API changes

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Data model changes

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