silhouette(): fix empty clusters case

[alyst]

Fixes #241

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (5d1ad38) 95.12% compared to head (3e6f01f) 95.13%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #242   +/-   ##
=======================================
  Coverage   95.12%   95.13%           
=======================================
  Files          18       18           
  Lines        1334     1335    +1     
=======================================
+ Hits         1269     1270    +1     
  Misses         65       65           

Impacted Files	Coverage Δ
src/silhouette.jl	98.36% <100.00%> (+0.02%)	⬆️

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