Fix KElbowVisualizer ValueError when null cluster is encountered

[busFred]

This PR fixes #1185 which ValueError is thrown when not all cluster center has at least one input data point assigned to it.

I have made the following changes:

1. Instead of reassigning labels = le.fit_transform(labels), modification were made to just call le.fit(labels) without reassigning input argument labels.

Sample Code and Plot

import numpy as np
from yellowbrick.cluster import distortion_score
X = np.array([[1,2],[3,4],[5,6]])
labels = np.array([1,3,3])
print(distortion_score(X, labels))
labels = np.array([0,1,1])
print(distortion_score(X, labels))

Console Output

4.000000000000001
4.000000000000001

TODOs and questions

Still to do: None

Questions for the @DistrictDataLabs/team-oz-maintainers:

• Please help me check this is the desired output as I might be wrong.

CHECKLIST

• [x] Is the commit message formatted correctly?
• [x] Have you noted the new functionality/bugfix in the release notes of the next release?

• [x] Included a sample plot to visually illustrate your changes?
• [x] Do all of your functions and methods have docstrings?
• [ ] Have you added/updated unit tests where appropriate?
• [x] Have you updated the baseline images if necessary?
• [ ] Have you run the unit tests using pytest?
• [x] Is your code style correct (are you using PEP8, pyflakes)?
• [x] Have you documented your new feature/functionality in the docs?

• [ ] Have you built the docs using make html?

[bbengfort]

@busFred thank you for contributing to Yellowbrick! I'm running some other tests right now, but will review shortly.

[codecov-commenter]

Codecov Report

Merging #1186 (ce177c5) into develop (07ef358) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1186   +/-   ##
========================================
  Coverage    90.44%   90.44%           
========================================
  Files           90       90           
  Lines         5076     5076           
========================================
  Hits          4591     4591           
  Misses         485      485           

Impacted Files	Coverage Δ
yellowbrick/cluster/elbow.py	98.49% <100.00%> (ø)

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