fix: declare types as nullable in Converters

Author: afranken

server/src/main/kotlin/com/adobe/testing/s3mock/controller/ChecksumModeHeaderConverter.kt

@@ -24,7 +24,7 @@ import org.springframework.core.convert.converter.Converter
  * Example:  x-amz-checksum-mode: ENABLED
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html)
  */
-open class ChecksumModeHeaderConverter : Converter<String, ChecksumMode> {
+open class ChecksumModeHeaderConverter : Converter<String, ChecksumMode?> {
   override fun convert(source: String): ChecksumMode? {
     return ChecksumMode.fromValue(source)
   }

server/src/main/kotlin/com/adobe/testing/s3mock/controller/HttpRangeHeaderConverter.kt

@@ -18,7 +18,7 @@ package com.adobe.testing.s3mock.controller
 import org.springframework.core.convert.converter.Converter
 import org.springframework.http.HttpRange
 
-open class HttpRangeHeaderConverter : Converter<String, HttpRange> {
+open class HttpRangeHeaderConverter : Converter<String, HttpRange?> {
   override fun convert(source: String): HttpRange? =
     HttpRange.parseRanges(source).firstOrNull()
 }

server/src/main/kotlin/com/adobe/testing/s3mock/controller/ObjectCannedAclHeaderConverter.kt

@@ -24,7 +24,7 @@ import org.springframework.core.convert.converter.Converter
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/userguide/acl-overview.html)
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/userguide/acl-overview.html#canned-acl)
  */
-open class ObjectCannedAclHeaderConverter : Converter<String, ObjectCannedACL> {
+open class ObjectCannedAclHeaderConverter : Converter<String, ObjectCannedACL?> {
   override fun convert(source: String): ObjectCannedACL? {
     return ObjectCannedACL.fromValue(source)
   }

server/src/main/kotlin/com/adobe/testing/s3mock/controller/ObjectOwnershipHeaderConverter.kt

@@ -26,7 +26,7 @@ import org.springframework.core.convert.converter.Converter
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/API/API_OwnershipControlsRule.html)
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/userguide/about-object-ownership.html)
  */
-open class ObjectOwnershipHeaderConverter : Converter<String, ObjectOwnership> {
+open class ObjectOwnershipHeaderConverter : Converter<String, ObjectOwnership?> {
   override fun convert(source: String): ObjectOwnership? {
     return ObjectOwnership.fromValue(source)
   }

server/src/main/kotlin/com/adobe/testing/s3mock/controller/RegionConverter.kt

@@ -24,7 +24,7 @@ import org.springframework.core.convert.converter.Converter
  * Example: bucket-region: us-east-1
  * [API Reference](https://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region)
  */
-open class RegionConverter : Converter<String, Region> {
+open class RegionConverter : Converter<String, Region?> {
   override fun convert(source: String): Region? {
     return Region.fromValue(source)
   }

server/src/main/kotlin/com/adobe/testing/s3mock/controller/TaggingHeaderConverter.kt

@@ -32,7 +32,7 @@ import tools.jackson.dataformat.xml.XmlMapper
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/API/API_PutObject.html)
  * [API Reference](https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html)
  */
-class TaggingHeaderConverter(private val xmlMapper: XmlMapper) : Converter<String, List<Tag>> {
+class TaggingHeaderConverter(private val xmlMapper: XmlMapper) : Converter<String, List<Tag>?> {
 
   override fun convert(source: String): List<Tag>? {
     val value = source.trim()