docstr: Take list of strings from multiple input nodes and join them
  into a single one returned in output. Since `input_str`
  is Variadic, we know we'll receive a List[List[str]].
code: def run(self, inputs: Variadic[List[str]]):
      """
      Take list of strings from multiple input nodes and join them
      into a single one returned in output. Since `input_str`
      is Variadic, we know we'll receive a List[List[str]].
      """
      retval: List[str] = []
      for list_of_strings in inputs:
          retval += list_of_strings
  
      return {"output": retval}
  
file_path: haystack/haystack/testing/sample_components/joiner.py
imports: from typing import List
  from haystack.core.component import component
  from haystack.core.component.types import Variadic
label_desc: Use image node_id 1 for calling the StringListJoiner obj's underlying member method code with example usage: obj.run(inputs) and returns: dict