Improvement suggestion: Error message when Embeddable has a missing field

[Nahuel92]

Issue description

Hi there!

I would like to suggest an improvement on how Micronaut Data handles error messages for the following use case.

When using a validated @Embeddable , if:

• It has a mandatory field
• The result set has a null value for that field

The error message is a bit confusing, as it says that the whole @Embeddable is null/missing. Take for example these data models:

@MappedEntity("my_entity")
public record MyEntity(
        @MappedProperty("id") @Id @GeneratedValue @Nullable Long id,
        @Relation(Relation.Kind.EMBEDDED) @Valid @NotNull AuditFields auditFields) {
}

@Embeddable
public record AuditFields(
        @MappedProperty("created_on") @NotNull LocalDate createdOn,
        @MappedProperty("created_by") @NotBlank String createdBy) {
}

If the DB has a missing/null createdBy field, for instance, this is the error message throw by the repository:

Error instantiating entity [io.github.nahuel92.MyEntity]: Null argument specified for [auditFields]. If this argument is allowed to be null annotate it with @Nullable

This can be improved by making the error message something like this:

Error instantiating entity [io.github.nahuel92.MyEntity]: Null argument specified for [auditFields.createdBy]. If this argument is allowed to be null annotate it with @Nullable

Here's an example app where you can see this in action:

https://github.com/Nahuel92/poc-projects/tree/master/micronaut/micronaut-data-embedded-misleading-error-message

Thanks!