fix: object of type NoneType has no len() - PotentialArithmOverflow detector fail

[shortdoom]

Note: Sorry for the mess, rebase somehow went wrong on my side in the old PR

Small fix to get rid of detector failing completely for some contracts. This is annoying because single detector fail stops the execution of other detectors.

Example error:

ERROR:root:Slither detection failed: object of type 'NoneType' has no len()
Traceback (most recent call last):
  File "/home/user/Github/Local/forgAi/core/analyze.py", line 670, in run_analysis
    self.run_slither_scan()
  File "/home/user/Github/Local/forgAi/core/analyze.py", line 556, in run_slither_scan
    slither, results_detectors, counter = run_all_detectors(
  File "/home/user/Github/Local/forgAi/core/utils.py", line 58, in run_all_detectors
    detector_resultss = slither.run_detectors()
  File "/home/user/Github/Local/forgAi/venv/lib/python3.10/site-packages/slither/slither.py", line 241, in run_detectors
    results = [d.detect() for d in self._detectors]
  File "/home/user/Github/Local/forgAi/venv/lib/python3.10/site-packages/slither/slither.py", line 241, in <listcomp>
    results = [d.detect() for d in self._detectors]
  File "/home/user/Github/Local/forgAi/venv/lib/python3.10/site-packages/slither/detectors/abstract_detector.py", line 203, in detect
    for r in [output.data for output in self._detect()]:
  File "/home/user/Github/Local/forgAi/venv/lib/python3.10/site-packages/slitherin/detectors/potential_arith_overflow.py", line 110, in _detect
    vulnerable_expressions = self._find_vulnerable_expressions(f)
  File "/home/user/Github/Local/forgAi/venv/lib/python3.10/site-packages/slitherin/detectors/potential_arith_overflow.py", line 89, in _find_vulnerable_expressions
    if len(irs) > 0 and isinstance(irs[-1], ops.Return) and len(fun.return_type) == 1 and str(fun.return_type[0]) in INT_TYPES: # @todo currently works only with single returns
TypeError: object of type 'NoneType' has no len()

Example contract to test with (without the patch): 0x61d25e9c65fb3bd2aa54e426b3044020dd339b8d