avoid npe when unable to parse config

Author: SylvainJuge

custom/src/main/java/co/elastic/otel/declarativeconfig/ElasticDeclarativeConfigurationCustomizer.java

@@ -62,6 +62,10 @@ public class ElasticDeclarativeConfigurationCustomizer
   public void customize(DeclarativeConfigurationCustomizer customizer) {
     customizer.addModelCustomizer(
         model -> {
+          if(model == null){
+            model = new OpenTelemetryConfigurationModel();
+          }
+
           customizeResources(model);
           customizeUserAgent(model);
           return model;