Add an option that allows developers to choose whether to include unfilled user inputs when submitting the form

Description

Add an option that allows developers to choose whether to include unfilled user inputs when submitting the form. It could be a new command tag, or a new parameter in the submit button tag.

Do not write null values for radio, dropdown, or date fields into the response anymore (to avoid downstream consumers expecting a String but receiving null).

I have the following comment regarding the above decision: we should allow developers to decide whether to write null values or exclude those fields in the JSON structure. This decision should be made by the app developers, who know the best way to generate the JSON structure based on the app's business logic. The decision of:

• Skipping fields and not generating them, or
• Generating them with null values, or
• Generating them with empty strings or any other default value (0 for integers or 1970-01-01 for dates),

should be made by the developer.

If all agree, this could be an enhancement for the widget: a new non-critical issue/ticket.

Originally posted by @srodriguez142857 in https://github.com/anusii/markdown_widget_builder/issues/101#issuecomment-4404205102