Improve if statement on Prefilling Mapping

dmp-backend/web/src/main/java/eu/eudat/logic/mapper/prefilling/PrefillingMapper.java

@@ -86,7 +86,7 @@ public class PrefillingMapper {
             }
             JsonNode valueNode = mapper.readTree(value);
             List<String> parsedValues = new ArrayList<>();
-            if (valueNode.isArray()) {
+            if (valueNode.isArray() && (!valueNode.get(0).isTextual() || !valueNode.get(0).isNull())) {
                 if (prefillingMapping.getSubSource() == null || prefillingMapping.getSubSource().isEmpty()) {
                     throw new IllegalArgumentException("Source value is an array but no subSource field have been set");
                 }