code: def test_pipeline_builder_merge_empty():
      builder_one_to_one = PipelineBuilder().add_sequence(
          "operation_a", "operation_f"
      )
  
      # empty left, empty right, empty both
      assert builder_one_to_one == merge_pipeline_builders(
          builder_one_to_one, PipelineBuilder()
      )
      assert builder_one_to_one == merge_pipeline_builders(
          PipelineBuilder(), builder_one_to_one
      )
      assert (
          len(
              merge_pipeline_builders(
                  PipelineBuilder(), PipelineBuilder()
              ).heads
          )
          == 0
      )
  
imports: from copy import copy
  from fedot.core.pipelines.pipeline_builder import PipelineBuilder, merge_pipeline_builders
  from fedot.core.pipelines.node import PipelineNode
  from fedot.core.pipelines.pipeline import Pipeline
  from test.unit.dag.test_graph_utils import graphs_same
label_desc: Use image node_id 8 for calling a global function with example usage: test_pipeline_builder_merge_empty() without return types