fix: pylint semantics

Author: f-allian

causal_testing/estimation/genetic_programming_regression_fitter.py

@@ -391,6 +391,7 @@ def mutate(self, expression: gp.PrimitiveTree) -> gp.PrimitiveTree:
 
         :return: The mutated expression.
         """
+        mutated = None
         choice = random.randint(1, 3)
         if choice == 1:
             mutated = gp.mutNodeReplacement(self.toolbox.clone(expression), self.pset)