Don't show a slider for a "flat" dimension

[Carifio24]

This is perhaps a bit of an edge case, but currently if there's a "flat" dimension in a dataset (i.e., the shape at that index is 1) that's not the x/y (and z if you're in the volume viewer) attribute, then you'll get a slice widget with no slider (since then high=0 and the usual set of buttons that then don't do anything. See the screenshot below.

This looks/feels kinda janky (I thought the slider was broken until I realized that the dataset I was looking at was "flat"), so what I propose here is that we just put in None in the list of sliders for that dimension - i.e. don't show a slider.

A quick sample dataset if one wants to reproduce this (run in the glue terminal):

from glue.core import Data
import numpy as np

x = np.arange(24).reshape((6, 4, 1))
d = Data(x=x, label="Flat Cube")
dc.append(d)

What this currently looks like:

[dhomeier]

Looks sensible to me.