solved an issue where missing dev dependencies were added to the list as None

[fpgmaas]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

[codecov-commenter]

Codecov Report

Merging #99 (0ec6f12) into main (f6d8a4b) will decrease coverage by 1.0%.
The diff coverage is 29.1%.

@@           Coverage Diff           @@
##            main     #99     +/-   ##
=======================================
- Coverage   95.7%   94.6%   -1.1%     
=======================================
  Files         30      30             
  Lines       1055    1073     +18     
=======================================
+ Hits        1010    1016      +6     
- Misses        45      57     +12     

Impacted Files	Coverage Δ
deptry/issue_finders/misplaced_dev.py	24.3% <10.5%> (-9.1%)	⬇️
tests/test_missing_dependencies_finder.py	100.0% <100.0%> (ø)

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