zimcheck: Test if the link is empty before testing its content.

[mgautierfr]

Fix #279

[codecov]

Codecov Report

Merging #280 (04dedc6) into master (a26a450) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #280   +/-   ##
=======================================
  Coverage   51.13%   51.13%           
=======================================
  Files          21       21           
  Lines        1936     1936           
  Branches     1145     1146    +1     
=======================================
  Hits          990      990           
  Misses        944      944           
  Partials        2        2           

Impacted Files	Coverage Δ
src/zimcheck/checks.cpp	97.72% <100.00%> (ø)

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