Fix NPE in ResourceHttpMessageConverter

ResourceHttpMessageConverter tries to use the filename to determine the
media type, but for Resource implementations such as ByteArrayResource
it is null, which causes NullPointerException. The fix checks whether
getFilename returns null before attempting to determine the media type
by it.

Issue: SPR-10848

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

@@ -138,6 +138,9 @@ public class ResourceHttpMessageConverter extends AbstractHttpMessageConverter<R
 		}
 
 		public static MediaType getMediaType(Resource resource) {
+			if(resource.getFilename() == null) {
+				return null;
+			}
 			String mediaType = fileTypeMap.getContentType(resource.getFilename());
 			return (StringUtils.hasText(mediaType) ? MediaType.parseMediaType(mediaType) : null);
 		}