code: def test_fstring_with_no_vars():
      fstring = FString(
          template="No variables in this template.", variables=[]
      )
      output = fstring.run()
      assert output == {"string": "No variables in this template."}
  
imports: import pytest
  from haystack.testing.sample_components import FString
label_desc: Use image node_id 2 for calling a global function with example usage: test_fstring_with_no_vars() without return types