bug(steps): handle col with one unique value in Scale* step

[jitingxu1]

similar issue to #118

When scaling, we need to calculate the min and max for ScaleMinMax, or the standard deviation for ScaleStandard.

If a column has only one unique value, the standard deviation (std) will be 0, or max - min will be 0.

This will result in the entire column being transformed to None.