Fix AbstractJackson2HttpMessageConverter nullness

This commit makes AbstractJackson2HttpMessageConverter#getObjectMappersForType
return value non nullable as an empty map is returned in case of no
registrations.

Closes gh-34811

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -184,7 +184,6 @@ public void registerObjectMappersForType(Class<?> clazz, Consumer<Map<MediaType,
 	 * or empty if in case of no registrations for the given class.
 	 * @since 5.3.4
 	 */
-	@Nullable
 	public Map<MediaType, ObjectMapper> getObjectMappersForType(Class<?> clazz) {
 		for (Map.Entry<Class<?>, Map<MediaType, ObjectMapper>> entry : getObjectMapperRegistrations().entrySet()) {
 			if (entry.getKey().isAssignableFrom(clazz)) {