title: Geopandas userdefined color scheme drops colors
body: <p>I expected to get a legend of the three colors green, yellow and red,
  even if the bottom range is empty (no numbers below 10).
  Instead GeoPandas drops the yellow color and uses green twice.</p>
  <p>Is this a bug or do I miss a parameter?</p>
  <pre><code>import pandas as pd
  import geopandas
  from matplotlib.colors import ListedColormap
  
  colors = ['green', 'yellow', 'red']
  bins = [10, 30]
  numbers = [15, 25, 35, 35, 55]
  
  ny = geopandas.read_file(geopandas.datasets.get_path('nybb'))
  numbers = pd.Series(numbers, name='numbers')
  ny = pd.concat([ny, numbers], axis=1)
  ny.plot(
      legend=True,
      column='numbers',
      scheme=&quot;user_defined&quot;,
      cmap = ListedColormap(colors),
      classification_kwds={'bins': bins},
  )
  </code></pre>
  <p><a href="https://i.stack.imgur.com/f6ZqU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f6ZqU.png" alt="Output of the plot" /></a></p>