Fix the object_type return value for the None case

[kounelisagis]

Following the changes in #2049, object_type now returns the string 'None' instead of the <class 'NoneType'> object. This PR fixes the issue and adds a new test for this case.

The output of the new test with the old object_type method:

=========================================================================== test session starts ============================================================================
platform darwin -- Python 3.11.9, pytest-8.3.1, pluggy-1.5.0
rootdir: /Users/agis/TileDB_workspace/TileDB-Py
configfile: pyproject.toml
plugins: hypothesis-6.108.4
collected 6 items / 5 deselected / 1 selected                                                                                                                              

tiledb/tests/test_util.py::UtilTest::test_object_type FAILED                                                                                                         [100%]

================================================================================= FAILURES =================================================================================
________________________________________________________________________ UtilTest.test_object_type _________________________________________________________________________

self = <tiledb.tests.test_util.UtilTest object at 0x106c8c650>

    def test_object_type(self):
        uri = self.path("test_object_type")
    
        # None case
>       self.assertIsNone(tiledb.object_type(uri))

tiledb/tests/test_util.py:173: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../anaconda3/envs/py11tiledb/lib/python3.11/contextlib.py:301: in helper
    return _GeneratorContextManager(func, args, kwds)
../../anaconda3/envs/py11tiledb/lib/python3.11/contextlib.py:105: in __init__
    self.gen = func(*args, **kwds)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tiledb.tests.test_util.UtilTest object at 0x106c8c650>, a1 = 'None'

    @contextlib.contextmanager
    def assertIsNone(self, a1):
>       assert a1 is None
E       AssertionError

tiledb/tests/common.py:183: AssertionError
========================================================================= short test summary info ==========================================================================
FAILED tiledb/tests/test_util.py::UtilTest::test_object_type - AssertionError
===================================================================== 1 failed, 5 deselected in 0.17s ======================================================================

---

[sc-55930]