Make BindingBuilder.asRequired use HasValue.isEmpty

[sissbruecker]

Describe your motivation

Currently BindingBuilder.asRequired creates a validator that compares the component's value against its HasValue.getEmptyValue. In some cases that is not useful, as a component might have multiple empty states. Consider the RichTextEditor.isEmpty implementation for example, which not only checks whether the value is an empty string, but also whether there are only empty HTML tags.

Describe the solution you'd like

Make BindingBuilder.asRequired use HasValue.isEmpty to test whether the field has a value or not. That would allow components to have a custom HasValue.isEmpty implementation which would be respected by the validator.

At least at the moment, it looks like the respective default implementations of isEmpty do the same as the validator created in asRequired.

Describe alternatives you've considered

A hacky workaround to make this work in a component would be to customize HasValue.getEmptyValue to return the actual component value if HasValue.isEmpty returns true, so that the validator is triggered. However that might also require overriding HasValue.clear to not use HasValue.getEmptyValue and instead use some constant value.

Alternatively, users could create custom validators instead of relying on the default validator created by asRequired.

[knoobie]

Might be related to #19847

[TatuLund]

A hacky workaround to make this work in a component would be to customize HasValue.getEmptyValue to return the actual component value if HasValue.isEmpty returns true, so that the validator is triggered.

Example of that workaround can be found here:

https://gist.github.com/TatuLund/e4a66e42a00cb0e4173d770ee5f870e8