Fix type error when concatenating None

[marcorosa]

Sometimes the mutator returns a result =None. In such case, concatenating a result=None with the prompt string raises an exception (TypeError, see below)

Traceback (most recent call last):
...
  File "...", line 53, in run
    mutated_results = self.mutate_policy.mutate_single(seed)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../venv/lib/python3.11/site-packages/gptfuzzer/fuzzer/mutator.py", line 216, in mutate_single
    results = [result + prompt_node.prompt for result in results]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../venv/lib/python3.11/site-packages/gptfuzzer/fuzzer/mutator.py", line 216, in <listcomp>
    results = [result + prompt_node.prompt for result in results]
               ~~~~~~~^~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

This PR fixes the TypeError concatenating the prompt string with either result value (if not None) or an empty string (if result is =None)