CAMEL-6674: Add allowNull option to @Converter for use-cases where a converter may return null as valid response. This fixes some converters for NodeList to Node which didnt work correctly before.

Conflicts:
	camel-core/src/main/java/org/apache/camel/converter/jaxp/XmlConverter.java
	components/camel-dozer/src/main/java/org/apache/camel/converter/dozer/DozerTypeConverter.java
Fixed CS