All three test cases are raising a VallueError now:

C:\py\cpython3.8>python -c "import os; os.spawnl(os.P_WAIT, '')"
Running Debug|x64 interpreter...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\py\cpython3.8\\lib\os.py", line 931, in spawnl
    return spawnv(mode, file, args)
ValueError: spawnv() arg 2 cannot be empty

C:\py\cpython3.8>python -c "import os; os.spawnl(os.P_WAIT, '__not_existing_path__')"
Running Debug|x64 interpreter...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\py\cpython3.8\\lib\os.py", line 931, in spawnl
    return spawnv(mode, file, args)
ValueError: spawnv() arg 2 cannot be empty

C:\py\cpython3.8>python -c "import os; os.spawnl(os.P_WAIT, '__not_existing_path__', '')"
Running Debug|x64 interpreter...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\py\cpython3.8\\lib\os.py", line 931, in spawnl
    return spawnv(mode, file, args)
ValueError: spawnv() arg 2 first element cannot be empty


But the following one still causes a crash:

  python -c "import os; os.spawnl(os.P_WAIT, '', 'non-empty')"