Check for the len in X before using to print training related log

[oneraghavan]

@kmike This will work.

[codecov-io]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 50%.

@@          Coverage Diff           @@
##           master     #29   +/-   ##
======================================
  Coverage    92.6%   92.6%           
======================================
  Files           8       8           
  Lines         257     257           
  Branches       37      37           
======================================
  Hits          238     238           
  Misses         15      15           
  Partials        4       4

Impacted Files	Coverage Δ
sklearn_crfsuite/estimator.py	95.45% <50%> (ø)	⬆️

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