Fix os.path.exists(None) issue:

```
______________________ ERROR collecting test_nvjitlink.py ______________________
tests/test_nvjitlink.py:62: in <module>
    not check_nvjitlink_usable(), reason="nvJitLink not usable, maybe not installed or too old (<12.3)"
tests/test_nvjitlink.py:58: in check_nvjitlink_usable
    return inner_nvjitlink._inspect_function_pointer("__nvJitLinkVersion") != 0
cuda/bindings/_internal/nvjitlink.pyx:257: in cuda.bindings._internal.nvjitlink._inspect_function_pointer
    ???
cuda/bindings/_internal/nvjitlink.pyx:260: in cuda.bindings._internal.nvjitlink._inspect_function_pointer
    ???
cuda/bindings/_internal/nvjitlink.pyx:208: in cuda.bindings._internal.nvjitlink._inspect_function_pointers
    ???
cuda/bindings/_internal/nvjitlink.pyx:102: in cuda.bindings._internal.nvjitlink._check_or_init_nvjitlink
    ???
cuda/bindings/_internal/nvjitlink.pyx:59: in cuda.bindings._internal.nvjitlink.load_library
    ???
/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/cuda/bindings/path_finder.py:312: in get_cuda_paths
    "nvvm": _get_nvvm_path(),
/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/cuda/bindings/path_finder.py:285: in _get_nvvm_path
    by, path = _get_nvvm_path_decision()
/opt/hostedtoolcache/Python/3.13.2/x64/lib/python3.13/site-packages/cuda/bindings/path_finder.py:96: in _get_nvvm_path_decision
    if os.path.exists(nvvm_ctk_dir):
<frozen genericpath>:19: in exists
    ???
E   TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType
```

Author: rwgk

cuda_bindings/cuda/bindings/path_finder.py

@@ -93,7 +93,7 @@ def _get_nvvm_path_decision():
     ]
     # need to ensure nvvm dir actually exists
     nvvm_ctk_dir = get_system_ctk(*_nvvm_lib_dir())
-    if os.path.exists(nvvm_ctk_dir):
+    if nvvm_ctk_dir and os.path.exists(nvvm_ctk_dir):
         options.append(("System", nvvm_ctk_dir))
 
     by, path = _find_valid_path(options)