Avoid returnin nan values for Precision@K

[osma]

Fixes #247

[codecov]

Codecov Report

Merging #248 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   99.24%   99.24%   +<.01%     
==========================================
  Files          48       48              
  Lines        2249     2252       +3     
==========================================
+ Hits         2232     2235       +3     
  Misses         17       17

Impacted Files	Coverage Δ
annif/eval.py	100% <100%> (ø)	⬆️

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[osma]

This pull request introduces 1 alert when merging 03571b0 into 3804bf9 - view on LGTM.com

new alerts:

• 1 for Unused local variable

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Comment posted by LGTM.com