fix: Fire validated event only when valid or validation changes

[TatuLund]

Fixes vaadin/flow-components#4986

[vursen]

This optimization doesn't appear correct. While the field remains invalid, its invalidity can be caused by different reasons, requiring different error messages.

Example:

1. Enter foo into a required date-picker => invalid due to bad input
2. Remove "foo" => invalid due to an empty value

[vursen]

Potentially yes, but in our current implementation the user indication is empty error message, and field shakes and goes invalid state

This will become problematic once the error message support is added, see vaadin/flow-components#4618.

Another problem is that the optimization doesn't appear to work when the field becomes invalid due to a failed Binder validator. From the client's perspective, the field will be considered valid, so it will keep sending validated events on every key press until the validator eventually succeeds.

Therefore, relying exclusively on the client-side validity in such optimizations isn't reliable.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[vursen]

I'm closing the PR due to the problems mentioned above.