code: public SourceState getPreviousDatasetState(String datasetUrn) {
      if (!this.previousDatasetStatesByUrns.containsKey(datasetUrn)) {
        return null;
      }
      return new ImmutableSourceState(this.previousDatasetStatesByUrns.get(datasetUrn));
    }
docs: Get the state (in the form of a {@link SourceState}) of a dataset identified by a dataset URN
  of the previous job run. Useful when dataset state store is enabled and we want to load the latest
  state of a global watermark dataset.
  
  @param datasetUrn the dataset URN
  @return the dataset state (in the form of a {@link SourceState}) of the previous job run
  or {@code null} if no previous dataset state is found for the given dataset URN