code: def test_generator_empty():
      graphs = [
          example_graph_random(feature_size=2, n_nodes=4),
          example_graph_random(
              feature_size=2, node_types=0, edge_types=0
          ),
      ]
  
      with pytest.raises(
          ValueError,
          match="graphs: expected every graph to be non-empty, found graph with no nodes",
      ):
          generator = PaddedGraphGenerator(graphs=graphs)
  
      generator = PaddedGraphGenerator(graphs=graphs[:1])
      with pytest.raises(
          ValueError,
          match="graphs: expected every graph to be non-empty, found graph with no nodes",
      ):
          seq = generator.flow(graphs)
  
imports: from stellargraph.core.graph import *
  from stellargraph.mapper.padded_graph_generator import PaddedGraphGenerator, PaddedGraphSequence
  import numpy
  import pytest
  from ..test_utils.graphs import example_graph_random, example_graph, example_hin_1
label_desc: Use image node_id 6 for calling a global function with example usage: test_generator_empty() without return types