Allow area search area description to be empty

[EmiliaMakelaVincit]

Taiga issue #1864

[vinikvuo]

Run black and isort then it is good to go

[codecov-commenter]

Codecov Report

Merging #521 (5d8d157) into develop (d8fb17a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #521   +/-   ##
========================================
  Coverage    75.73%   75.74%           
========================================
  Files          439      440    +1     
  Lines        23166    23170    +4     
  Branches      2327     2327           
========================================
+ Hits         17545    17549    +4     
  Misses        5182     5182           
  Partials       439      439           

Files Changed	Coverage Δ
...grations/0029_alter_areasearch_description_area.py	100.00% <100.00%> (ø)
plotsearch/models/plot_search.py	90.10% <100.00%> (ø)

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