Fix NPE in AbstractMessageConverterMethodProcessor

Issue: SPR-9868

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/AbstractMessageConverterMethodProcessor.java

@@ -141,9 +141,8 @@ public abstract class AbstractMessageConverterMethodProcessor extends AbstractMe
 			}
 		}
 
-		selectedMediaType = selectedMediaType.removeQualityValue();
-
 		if (selectedMediaType != null) {
+			selectedMediaType = selectedMediaType.removeQualityValue();
 			for (HttpMessageConverter<?> messageConverter : messageConverters) {
 				if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
 					((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);