Catch and test case without value column

[Anaphory]

Fixes #336

[codecov]

Codecov Report

Merging #341 (e3c6ea7) into master (16175c9) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   79.69%   79.73%   +0.04%     
==========================================
  Files          43       43              
  Lines        4270     4274       +4     
  Branches     1082     1082              
==========================================
+ Hits         3403     3408       +5     
  Misses        663      663              
+ Partials      204      203       -1     

Impacted Files	Coverage Δ
src/lexedata/importer/excel_long_format.py	74.74% <100.00%> (+1.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 16175c9...e3c6ea7. Read the comment docs.