ExtraCoords half works if not attached to a NDCube

[Cadair]

Describe the bug

I came across this due to this conversation. We should decide what we want the behaviour to be for a standalone ExtraCoords object.

To Reproduce

import astropy.units as u
from ndcube import ExtraCoords

ec = ExtraCoords.from_lookup_tables(('hello', 'bye'),
                                    (1, 1),
                                    (list(range(3)) * u.m,
                                    list(range(3)) * u.keV,
                                    )
                                   )
ec.wcs

<WCS(output_frame=CompositeFrame, input_frame=PixelFrame, forward_transform=Model: CompoundModel
Inputs: ('x0', 'x1')
Outputs: ('y0', 'y1')
Model set size: 1
Expression: [0] & [1]
Components: 
    [0]: <Tabular1D(points=(<Quantity [0., 1., 2.] pix>,), lookup_table=[0. 1. 2.] m)>

    [1]: <Tabular1D(points=(<Quantity [0., 1., 2.] pix>,), lookup_table=[0. 1. 2.] keV)>
Parameters:)>

ec.mapping

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[6], line 1
----> 1 ec.mapping

File ~/Git/ndcube/ndcube/extra_coords/extra_coords.py:264, in ExtraCoords.mapping(self)
    261 # The mapping is from the array index (position in the list) to the
    262 # pixel dimensions (numbers in the list)
    263 lts = [list([lt[0]] if isinstance(lt[0], Integral) else lt[0]) for lt in self._lookup_tables]
--> 264 converter = partial(convert_between_array_and_pixel_axes, naxes=len(self._ndcube.shape))
    265 pixel_indicies = [list(converter(np.array(ids))) for ids in lts]
    266 return tuple(reduce(list.__add__, pixel_indicies))

AttributeError: 'NoneType' object has no attribute 'shape'

Screenshots

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System Details

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Installation method

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