code: def test_null():
      print("Null model test")
      print("Create custom null model")
      (model, f_test) = regression_model_distribution(
          H2OGradientBoostingEstimator,
          upload_distribution(CustomDistributionGeneric, "null"),
      )
      print("rmse null model train:", model.rmse(valid=False))
      print("rmse null model valid:", model.rmse(valid=True))
  
imports: import sys
  import h2o
  from tests import pyunit_utils
  from h2o.utils.distributions import CustomDistributionGaussian, CustomDistributionBernoulli, CustomDistributionMultinomial, CustomDistributionGeneric
  from tests.pyunit_utils import regression_model_distribution, regression_model_default
  from tests.pyunit_utils import multinomial_model_default, multinomial_model_distribution
  from tests.pyunit_utils import binomial_model_default, binomial_model_distribution, check_model_metrics
  from h2o.estimators.gbm import H2OGradientBoostingEstimator
label_desc: Use image node_id 6 for calling a global function with example usage: test_null() without return types