LinearInterpolatedResampler fails on uncertainty of UnknownUncertainty type

When (unsuccessfully) trying to reproduce #647 I raised a different exception by modifying the docs' [example](https://specutils.readthedocs.io/en/stable/api/specutils.manipulation.LinearInterpolatedResampler.html):
```
input_spectrum = Spectrum1D(flux=[1, 3, 7, 6, 20] * u.mJy,
spectral_axis=[2, 4, 12, 16, 20] * u.nm,
uncertainty=[0.4, 0.5, 0.6, 0.8, 1])
resample_grid = [1, 5, 9, 13, 14, 17, 21, 22, 23] * u.nm
fluxc_resample = LinearInterpolatedResampler()

input_spectrum.uncertainty
UnknownUncertainty([0.4, 0.5, 0.6, 0.8, 1. ])
output_spectrum1D = fluxc_resample(input_spectra, resample_grid)

---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in
----> 1 output_spectrum1D = fluxc_resample(input_spectrum, resample_grid)

~/lib/python3.8/site-packages/specutils/manipulation/resample.py in __call__(self, orig_spectrum, fin_spec_axis)
37 Return the resulting `~specutils.Spectrum1D` of the resampling.
38 """
---> 39 return self.resample1d(orig_spectrum, fin_spec_axis)
40
41 @abstractmethod

~/lib/python3.8/site-packages/specutils/manipulation/resample.py in resample1d(self, orig_spectrum, fin_spec_axis)
321 unit=orig_spectrum.unit)
322
--> 323 return Spectrum1D(spectral_axis=fin_spec_axis,
324 flux=out_flux,
325 uncertainty=new_unc)

~/lib/python3.8/site-packages/specutils/spectra/spectrum1d.py in __init__(self, flux, spectral_axis, wcs, velocity_convention, rest_value, redshift, radial_velocity, **kwargs)
157 spectral_axis.shape[0], flux.shape[-1]))
158
--> 159 super(Spectrum1D, self).__init__(
160 data=flux.value if isinstance(flux, u.Quantity) else flux,
161 wcs=wcs, **kwargs)

~/lib/python3.8/site-packages/astropy/nddata/nddata.py in __init__(self, data, uncertainty, mask, wcs, meta, unit, copy)
232 self._unit = unit
233 # Call the setter for uncertainty to further check the uncertainty
--> 234 self.uncertainty = uncertainty
235
236 def __str__(self):

~/lib/python3.8/site-packages/astropy/nddata/nddata.py in uncertainty(self, value)
324 # to be saved as weakref but that's done by NDUncertainty
325 # setter).
--> 326 value.parent_nddata = self
327 self._uncertainty = value

~/lib/python3.8/site-packages/astropy/nddata/nduncertainty.py in parent_nddata(self, value)
231 unit_from_data = self._data_unit_to_uncertainty_unit(parent_unit)
232 try:
--> 233 unit_from_data.to(self.unit)
234 except UnitConversionError:
235 raise UnitConversionError("Unit {} of uncertainty "

AttributeError: 'NoneType' object has no attribute 'to'
```
Is there other functionality that would handle an `astropy.nddata.nduncertainty.UnknownUncertainty` correctly, or should this best be checked on initialising `Spectrum1D`?

Start with the docs example for LinearInterpolatedResampler and the resample1d entry point shown in the traceback. Reproduce the UnknownUncertainty case and inspect how the resulting uncertainty is passed to Spectrum1D; done means the example no longer raises AttributeError and the intended handling is covered by an appropriate test.

Written by the indexing model from the issue text.