OWNxFromDistances: Fix crash when the matrix is empty

[janezd]

Fixes #106.

[codecov-io]

Codecov Report

Merging #118 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
- Coverage   62.14%   62.09%   -0.05%     
==========================================
  Files          29       29              
  Lines        2800     2802       +2     
==========================================
  Hits         1740     1740              
- Misses       1060     1062       +2

Impacted Files	Coverage Δ
orangecontrib/network/widgets/OWNxFromDistances.py	25.09% <0%> (-0.2%)	⬇️

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