Do not print behavior form if there is nothing a plugin can do

Problem/Motivation

Behavior forms allow behavior plugins to specify and build (form) options that can be applied to Paragraph entities.
Sometimes, plugins do not have any form options or the global default behavior is applied.

Still, Paragraphs Experimental widget always adds at least an empty container form element (See https://cgit.drupalcode.org/paragraphs/tree/src/Plugin/Field/FieldWidget...) which results in needless markup on the page.

Proposed resolution

Unset initially created behavior plugin form element if a plugin responds with an empty renderable array.

Remaining tasks

User interface changes

API changes

Data model changes