text: Title: [BUG] Index-based colormodes raise `ZeroDivisionError` for single-trace plots
  Body: All index-based colormodes raise a `ZeroDivisionError` when plotting a ridgeline plot with either a single trace or a single trace per row.
  
  The index-based colormodes are:
  
  ```python
  colormodes = [
      "row-index",
      "trace-index",
      "trace-index-row-wise",
  ]
  ```
  
  MVE:
  
  ```pycon
  >>> from ridgeplot import ridgeplot
  >>> ridgeplot(samples=[[1, 2, 2, 3]], colormode=colormodes[0])
  Traceback (most recent call last):
  ...
    File "[...]/ridgeplot/_color/interpolation.py", line 68, in _interpolate_row_index
      [((ctx.n_rows - 1) - ith_row) / (ctx.n_rows - 1)] * len(row)
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
  ZeroDivisionError: division by zero
  ```
  