feat: add falthyAsNull to nullable

src/items/nullable.ts

@@ -1,9 +1,17 @@
+import { parceable } from 'Schema'
 import SchemaItem from '../SchemaItem'
 import { SchemaInfer, ValidationResult } from '../types'
 
 export default class SchemaNullable<Type extends SchemaItem> extends SchemaItem<SchemaInfer<Type> | undefined> {
 	public constructor(public readonly child: Type) { super([child]) }
 
+	@parceable()
+	public falthyAsNull() {
+		this.addPreProcess((it) => !it ? undefined : it)
+
+		return this
+	}
+
 	public unwrap(): Type {
 		return this.child
 	}