Scale singleton vectors to [0]

Currently

```julia
julia> MeanStdScaling([1])
MeanStdScaling(1.0, NaN)
```

due to `Statistics.std` behaviour: `corrected=true` by default, which divides the sum of errors by N-1 instead of N, and N-1 is 0 for singletons.

It's an edge case, but could arise in feature engineering pipelines where the size of available data varies. I think for the purpose of feature engineering, a singleton vector should be standardized to `[0]` rather than `[NaN]`.