code: private String isNotNullStep(Step<?, ?> currentStep) {
          if (currentStep instanceof TraversalFilterStep<?>) {
              TraversalFilterStep<?> tfs = (TraversalFilterStep<?>) currentStep;
              List<?> c = tfs.getLocalChildren();
              if (c != null && c.size() == 1) {
                  Traversal.Admin<?, ?> a = (Traversal.Admin<?, ?>) c.iterator().next();
                  Step<?, ?> s = a.getEndStep();
                  if (a.getSteps().size() == 1 && s instanceof PropertiesStep<?>) {
                      PropertiesStep<?> ps = (PropertiesStep<?>) s;
                      String[] keys = ps.getPropertyKeys();
                      if (keys != null && keys.length == 1) {
                          return keys[0];
                      }
                  }
              }
          }
          return null;
      }
docs: if this is a has(property) step, returns the property key, otherwise returns null
  
  @param currentStep the step
  @return the property which should be not null