Fix failure to create Colormap for single-channel data

[will-moore]

This fixes the opening of images without a channel dimension that have color info in an omero metadata.

For images without a channel dimension, we convert

colormaps = [
   [[r,g,b], [r2,g2,b2]]
]

into

colormaps = [[r,g,b], [r2,g2,b2]]

but this is not handled by the subsequent code that tries to create ColorMaps. So it tries to create a ColorMap from [r,g,b] instead of [[r,g,b], [r2,g2,b2]].
To fix this, simply move the ColorMap creation code before the unwrapping of the list for single-channel images.

Since we only find this bug with v0.3 data that don't have a c dimension, it is not found on any public data.

To test, NB: needs ome/ome-zarr-py#114 to create 2D sample image:

$ ome_zarr create coinsimg
$ napari coinsimg

[codecov-commenter]

Codecov Report

Merging #15 (16a7a2e) into main (be5ff0d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   90.74%   90.74%           
=======================================
  Files           3        3           
  Lines         162      162           
=======================================
  Hits          147      147           
  Misses         15       15           

Impacted Files	Coverage Δ
napari_ome_zarr/_reader.py	85.88% <100.00%> (ø)

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