Allow behavior plugins don't provide any field in the behavior form

Problem/Motivation

With the addition of per plugin config for the behavior plugins some of them will not need to show any field in the behavior form that is displayed while editing paragraphs.
With the current implementation a behavior plugin MUST provide at least one field for the behavior form, which won't be always the case, otherwise there will be a white display of error.

Proposed resolution

Allow behavior plugins not adding any behavior field.

Remaining tasks

User interface changes

API changes

Data model changes