OptionalAttribute class

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
[Conditional("UNITY_EDITOR")]
public class OptionalAttribute : Attribute, _Attribute

Overrides the 'Reference Required by Default' rule to allow for null values. Has no effect if the rule is disabled. This attribute does not do anything unless you have Odin Validator and the 'Reference Required by Default' rule is enabled.

public OptionalAttribute()