New feature: Allow empty value for select form controls, based on user setting.

[vlad-ghita]

Use case

• have a required select field;
• select field must not pre-select any option;
• user must deliberately choose an option;

Current Bolt behaviour automatically selects first option if field is required.

Solution

Add a configuration setting called allow_empty which enables developer to force an empty option for select control, even when control is required.

allow_empty setting is available in following definitions:

• select field
• relations
• taxonomies

Image below shows definitions with these settings:

required: true
allow_empty: true

Testing

Manually tested these combinations:

required	allow_empty
none	none
none	true
none	false
true	none
true	true
true	false
false	none
false	true
false	false

Further improvements

• automatic validation. Given empty option is added automatically, Bolt can automatically validate select value and return an error.
• unit test

[bobdenotter]

Hi @vlad-ghita I'm going to review this PR. Thanks!

I've fixed a tiny CS issue directly in the PR. It's still complaining about a PHPStan error, but this is fixed upstream, already.