Fix: filter_difficulty now handles when trek has no difficulty value

[marcantoinedupre]

No description provided.

[cypress]

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Test summary

24 • 0 • 0 • 0

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Run details

Project	Geotrek-admin
Status	Passed
Commit	8d09ddc ℹ️
Started	Jan 19, 2023 2:02 PM
Ended	Jan 19, 2023 2:05 PM
Duration	03:20 💡
OS	Linux Ubuntu - 20.04
Browser	Electron 94

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[codecov]

Codecov Report

Base: 98.23% // Head: 98.23% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (2134c58) compared to base (8ac5f78).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3415   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         291      291           
  Lines       20789    20790    +1     
=======================================
+ Hits        20423    20424    +1     
  Misses        366      366           

Impacted Files	Coverage Δ
geotrek/trekking/parsers.py	100.00% <100.00%> (ø)

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