Fix ServletResponseHttpHeaders#get null handling

ServletResponseHttpHeaders#get should be annotated with `@Nullable` and
return null instead of a singleton list containing null when there is no
content type header.

Closes gh-32362

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -171,13 +171,15 @@ public class ServletServerHttpResponse implements ServerHttpResponse {
 		}
 
 		@Override
+		@Nullable
 		public List<String> get(Object key) {
 			Assert.isInstanceOf(String.class, key, "Key must be a String-based header name");
 
 			String headerName = (String) key;
 			if (headerName.equalsIgnoreCase(CONTENT_TYPE)) {
 				// Content-Type is written as an override so don't merge
-				return Collections.singletonList(getFirst(headerName));
+				String value = getFirst(headerName);
+				return (value != null ? Collections.singletonList(value) : null);
 			}
 
 			Collection<String> values1 = servletResponse.getHeaders(headerName);