fix(core): allow null/undefined values for required fields with defaults

[cyphercodes]

Problem

When a field is required (notNull) and has a default value, creating a new record without explicitly providing a value for that field would fail validation with a 'Required' error. This happened because the validation logic didn't account for the fact that a default value would be applied later in the record creation process.

Fixes #2771

Solution

Modified validateCellValueWithNotNull in the FieldCore class to check if the field has a default value before rejecting null/undefined inputs. If a default value is defined, the validation allows null/undefined to pass through, since the default will be applied later.

Changes

• Updated validateCellValueWithNotNull in packages/core/src/models/field/field.ts
• Added check for options.defaultValue before returning validation error for null/undefined values

Testing

This fix ensures that:

1. Required fields without defaults still reject null/undefined values
2. Required fields with defaults allow null/undefined values (default will be applied later)
3. Non-required fields continue to work as before

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