SpringBoot Fix nullability declaration in Kotlin samples

Some samples have mismatches in their nullability constraints. While this seems to be ignored by Kotlin 1.6.0, 1.6.20-RC is throwing an explicit exception:

error: warnings found and -Werror specified
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/data/nosql/couchbase/repositories/MyConverter.kt:23:2: warning: override 'fun convert(value: CouchbaseProperties?): Boolean' has incorrect nullability in its signature comparing with overridden 'fun convert(source: CouchbaseProperties): Boolean?'
        override fun convert(value: CouchbaseProperties?): Boolean {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/messaging/amqp/receiving/custom/MyMessageConverter.kt:25:2: warning: override 'fun toMessage(`object`: Any, messageProperties: MessageProperties): Message?' has incorrect nullability in its signature comparing with overridden 'fun toMessage(p0: Any, p1: MessageProperties): Message'
        override fun toMessage(`object`: Any, messageProperties: MessageProperties): Message? {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/messaging/amqp/receiving/custom/MyMessageConverter.kt:29:2: warning: override 'fun fromMessage(message: Message): Any?' has incorrect nullability in its signature comparing with overridden 'fun fromMessage(p0: Message): Any'
        override fun fromMessage(message: Message): Any? {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/messaging/jms/receiving/custom/MyMessageConverter.kt:25:2: warning: override 'fun toMessage(`object`: Any, session: Session): Message?' has incorrect nullability in its signature comparing with overridden 'fun toMessage(`object`: Any, session: Session): Message'
        override fun toMessage(`object`: Any, session: Session): Message? {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/messaging/jms/receiving/custom/MyMessageConverter.kt:29:2: warning: override 'fun fromMessage(message: Message): Any?' has incorrect nullability in its signature comparing with overridden 'fun fromMessage(message: Message): Any'
        override fun fromMessage(message: Message): Any? {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/web/servlet/springmvc/messageconverters/AdditionalHttpMessageConverter.kt:28:2: warning: override 'fun supports(clazz: Class<*>?): Boolean' has incorrect nullability in its signature comparing with overridden 'fun supports(clazz: Class<*>): Boolean'
        override fun supports(clazz: Class<*>?): Boolean {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/web/servlet/springmvc/messageconverters/AdditionalHttpMessageConverter.kt:33:2: warning: override 'fun readInternal(clazz: Class<*>?, inputMessage: HttpInputMessage): Any?' has incorrect nullability in its signature comparing with overridden 'fun readInternal(clazz: Class<out Any?>, inputMessage: HttpInputMessage): Any'
        override fun readInternal(clazz: Class<*>?, inputMessage: HttpInputMessage): Any? {
 ^
spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/web/servlet/springmvc/messageconverters/AdditionalHttpMessageConverter.kt:38:2: warning: override 'fun writeInternal(t: Any?, outputMessage: HttpOutputMessage): Unit' has incorrect nullability in its signature comparing with overridden 'fun writeInternal(t: Any, outputMessage: HttpOutputMessage): Unit'
        override fun writeInternal(t: Any?, outputMessage: HttpOutputMessage) {
 ^