fix: don't fail with NPE if there are no code list associations

This prevents a `NullPointerException` when a deegree configuration is
written with the Primitive option `All link where code lists are
assigned in hale` and there are no codelist associations in the
alignment.

io/plugins/eu.esdihumboldt.hale.io.deegree/src/eu/esdihumboldt/hale/io/deegree/mapping/MappingWriter.java

@@ -224,6 +224,10 @@ else if (config.getPrimitiveLinkMode()
 						.getSetting(CodeListAssociations.KEY_ASSOCIATIONS)
 						.as(CodeListAssociations.class);
 
+				if (associations == null) {
+					return null;
+				}
+
 				return associations.getAssociations().keySet().stream().map(e -> {
 					List<QName> names = e.getNames();
 					if (names.size() > 1) {