[Admin] Fix user locale handling in SolidusAdmin

[elia]

Summary

The current implementation was writing on the session key if the requested locale was empty and the session key was set.

Checklist

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[codecov]

Codecov Report

Merging #5303 (9ce3bc6) into nebulab/admin (7cd6dd5) will decrease coverage by 0.01%.
The diff coverage is 81.81%.

@@                Coverage Diff                @@
##           nebulab/admin    #5303      +/-   ##
=================================================
- Coverage          88.57%   88.56%   -0.01%     
=================================================
  Files                599      600       +1     
  Lines              14594    14604      +10     
=================================================
+ Hits               12926    12934       +8     
- Misses              1668     1670       +2     

Files Changed	Coverage Δ
...rollers/solidus_admin/controller_helpers/locale.rb	76.47% <76.47%> (ø)
...n/app/controllers/solidus_admin/base_controller.rb	100.00% <100.00%> (+10.52%)	⬆️
...ntrollers/solidus_admin/controller_helpers/auth.rb	100.00% <100.00%> (ø)
admin/lib/solidus_admin/preview.rb	93.33% <100.00%> (ø)

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