fix: form validators check and disabled prop

[hardikpthv]

What I did

1. fix(form-core): check for nullability for disabled prop
2. fix(form-core): make use of validatorName and type to check validator instance

[changeset-bot]

🦋 Changeset detected

Latest commit: 5eb7fc9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@lion/ui	Patch

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[hardikpthv]

Let me know if i need to add changeset :)

[okadurin]

Let me know if i need to add changeset :)

Please do :)

[meteabduraman]

hey @hardikpthv @tlouisse I noticed this PR from a while back changes how the ValidateMixin determines whether a Required validator is a Required validator (by checking .validatorName instead of looking at instanceof), but could it be that line 485 of ValidateMixin.js was not updated and it should have?

i don't see a reason to not use .validatorName there too 🤔 is there?