Fix Kotlin example for empty @DefaultValue constructor binding

Closes gh-32177

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/external-config.adoc

@@ -730,11 +730,10 @@ Default values can be specified using `@DefaultValue` on a constructor parameter
 The conversion service will be applied to coerce the `String` value to the target type of a missing property.
 
 Referring to the previous example, if no properties are bound to `Security`, the `MyProperties` instance will contain a `null` value for `security`.
-If you wish you return a non-null instance of `Security` even when no properties are bound to it, you can use an empty `@DefaultValue` annotation to do so:
+To make it contain a non-null instance of `Security` even when no properties are bound to it (when using Kotlin, this will require the `username` and `password` parameters of `Security` to be declared as nullable as they do not have default values), use an empty `@DefaultValue` annotation:
 
 include::code:nonnull/MyProperties[tag=*]
 
-
 NOTE: To use constructor binding the class must be enabled using `@EnableConfigurationProperties` or configuration property scanning.
 You cannot use constructor binding with beans that are created by the regular Spring mechanisms (for example `@Component` beans, beans created by using `@Bean` methods or beans loaded by using `@Import`)
 

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/externalconfig/typesafeconfigurationproperties/constructorbinding/nonnull/MyProperties.kt

@@ -23,10 +23,12 @@ import java.net.InetAddress
 
 @ConstructorBinding
 @ConfigurationProperties("my.service")
-class MyProperties(val isEnabled: Boolean, val remoteAddress: InetAddress,
-		@param:DefaultValue val security: Security) {
+// tag::code[]
+class MyProperties(val enabled: Boolean, val remoteAddress: InetAddress,
+		@DefaultValue val security: Security) {
 
-	class Security(val username: String, val password: String,
-		@param:DefaultValue("USER") val roles: List<String>)
+	class Security(val username: String?, val password: String?,
+			@param:DefaultValue("USER") val roles: List<String>)
 
-}
+}
+// end::code[]