Refine *HttpMessageConverter#getContentLength null safety

Closes gh-32333

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -113,6 +113,7 @@ public class ResourceHttpMessageConverter extends AbstractHttpMessageConverter<R
 	}
 
 	@Override
+	@Nullable
 	protected Long getContentLength(Resource resource, @Nullable MediaType contentType) throws IOException {
 		// Don't try to determine contentLength on InputStreamResource - cannot be read afterwards...
 		// Note: custom InputStreamResource subclasses could provide a pre-calculated content length!

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -561,6 +561,7 @@ public abstract class AbstractJackson2HttpMessageConverter extends AbstractGener
 	}
 
 	@Override
+	@Nullable
 	protected Long getContentLength(Object object, @Nullable MediaType contentType) throws IOException {
 		if (object instanceof MappingJacksonValue mappingJacksonValue) {
 			object = mappingJacksonValue.getValue();