Add additional explanation regarding zero as a label

[stefanhahmann]

• This was motivated by running into unexpected behaviour when using a label image and a measurement table that used 0 as a label, cf.: Label image disappears when performing clustering on timelapse data #269

[jo-mueller]

Hi @stefanhahmann ,

I think this should be good to go, thanks for spotting and opening the issue. Where I am concerned: Feel free to merge!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9398680) 76.38% compared to head (779cac0) 76.39%.
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
+ Coverage   76.38%   76.39%   +0.01%     
==========================================
  Files          16       16              
  Lines        1859     1860       +1     
==========================================
+ Hits         1420     1421       +1     
  Misses        439      439              

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[Cryaaa]

Since tests are now passing after updating from main this is good to go!