[Backport to 4.x][Fixes #9604] Report empty thumbnail in case a thumbnail has not been…

[afabiani]

… generated for a resource (#9613)

• -[Fixes #9604] Report empty thumbnail in case a thumbnail has not been generated for a resource

• • update tests

• -remove unused import

• -remove unused import

• • create Thumbnail Link if thumbnail_url exists
• • fix tests
• • fix pep8
• • add command to clean resources thumbs
• • fix lgtm
• • rename migration file
• • fix migration dependence

Co-authored-by: Alessio Fabiani [email protected] (cherry picked from commit 38cbd9dd752f188a4ac1f3cffb498ad0cd0e24eb)

Checklist

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[codecov[bot]]

Codecov Report

Merging #9810 (fa2e555) into 4.x (f4f4fbf) will increase coverage by 0.01%. The diff coverage is 83.14%.

@@            Coverage Diff             @@
##              4.x    #9810      +/-   ##
==========================================
+ Coverage   61.28%   61.29%   +0.01%     
==========================================
  Files         821      822       +1     
  Lines       50165    50204      +39     
  Branches     7726     7736      +10     
==========================================
+ Hits        30742    30771      +29     
- Misses      17742    17751       +9     
- Partials     1681     1682       +1