Handle null value when parsing kubevela string

This was the root cause of
eu-nebulous/optimiser-utility-evaluator#5

nebulous-requirements-extractor/src/main/java/eu/nebulouscloud/optimiser/kubevela/KubevelaAnalyzer.java

@@ -1,6 +1,7 @@
 package eu.nebulouscloud.optimiser.kubevela;
 
 
+import com.fasterxml.jackson.core.JsonParseException;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -494,6 +495,9 @@ public static Map<String, List<Requirement>> getBoundedRequirements(String kubev
      * @throws JsonProcessingException if kubevela does not contain valid YAML.
      */
     public static JsonNode parseKubevela(String kubevela) throws JsonProcessingException {
+        if (kubevela == null) {
+            throw new JsonParseException("The provided string value was null");
+        }
         return yamlMapper.readTree(kubevela);
     }