fix type error on inst to pd with single prop and expand True

[haakonvt]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
File /lib/python3.12/site-packages/IPython/core/formatters.py:347, in BaseFormatter.__call__(self, obj)
    345     method = get_real_method(obj, self.print_method)
    346     if method is not None:
--> 347         return method()
    348     return None
    349 else:

File /lib/python3.12/site-packages/cognite/client/data_classes/_base.py:239, in CogniteResource._repr_html_(self)
    238 def _repr_html_(self) -> str:
--> 239     return notebook_display_with_fallback(self)

File /lib/python3.12/site-packages/cognite/client/utils/_pandas_helpers.py:99, in notebook_display_with_fallback(inst, **kwargs)
     97     kwargs["expand_properties"] = True
     98 try:
---> 99     return inst.to_pandas(**kwargs)._repr_html_()
    100 except CogniteImportError:
    101     warnings.warn(
    102         "The 'cognite-sdk' depends on 'pandas' for pretty-printing objects like 'Asset' or 'DatapointsList' in "
    103         "(Jupyter) notebooks and similar environments. Consider installing it! Using fallback method.",
    104         UserWarning,
    105     )

File /lib/python3.12/site-packages/cognite/client/data_classes/data_modeling/instances.py:475, in Instance.to_pandas(self, ignore, camel_case, convert_timestamps, expand_properties, remove_property_prefix, **kwargs)
    470     else:
    471         warnings.warn(
    472             "Can't remove view ID prefix from expanded property rows as source was not unique",
    473             RuntimeWarning,
    474         )
--> 475 return pd.concat((col, prop_df.T.squeeze())).to_frame(name="value")

File /lib/python3.12/site-packages/pandas/core/reshape/concat.py:380, in concat(objs, axis, join, ignore_index, keys, levels, names, verify_integrity, sort, copy)
    377 elif copy and using_copy_on_write():
    378     copy = False
--> 380 op = _Concatenator(
    381     objs,
    382     axis=axis,
    383     ignore_index=ignore_index,
    384     join=join,
    385     keys=keys,
    386     levels=levels,
    387     names=names,
    388     verify_integrity=verify_integrity,
    389     copy=copy,
    390     sort=sort,
    391 )
    393 return op.get_result()

File /lib/python3.12/site-packages/pandas/core/reshape/concat.py:446, in _Concatenator.__init__(self, objs, axis, join, keys, levels, names, ignore_index, verify_integrity, copy, sort)
    443 objs, keys = self._clean_keys_and_objs(objs, keys)
    445 # figure out what our result ndim is going to be
--> 446 ndims = self._get_ndims(objs)
    447 sample, objs = self._get_sample_object(objs, ndims, keys, names, levels)
    449 # Standardize axis parameter to int

File /lib/python3.12/site-packages/pandas/core/reshape/concat.py:487, in _Concatenator._get_ndims(self, objs)
    482     if not isinstance(obj, (ABCSeries, ABCDataFrame)):
    483         msg = (
    484             f"cannot concatenate object of type '{type(obj)}'; "
    485             "only Series and DataFrame objs are valid"
    486         )
--> 487         raise TypeError(msg)
    489     ndims.add(obj.ndim)
    490 return ndims

TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid