code: def test_pipeline_builder_with_empty_inputs():
      # None inputs and empty pipelines
      assert PipelineBuilder().build() is None
      assert PipelineBuilder().add_node(None).build() is None
      assert (
          PipelineBuilder().grow_branches(None, None, None).build()
          is None
      )
      assert (
          PipelineBuilder().join_branches("operation_a").build() is None
      )  # empty join is empty
      assert PipelineBuilder().join_branches(None).build() is None
  
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 2 for calling a global function with example usage: test_pipeline_builder_with_empty_inputs() without return types