Dummy app generator: Only configure app/assets/javascripts if present

[mamhoff]

Newer versions of Rails generators do not generate the app/assets/javascripts directory. Let's not add it to the Sprockets config if it's not there.

[tvdeyen]

Thanks. We paired on this

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (61f675e) to head (ca1af55).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
core/lib/generators/spree/dummy/dummy_generator.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6227      +/-   ##
==========================================
- Coverage   88.80%   88.80%   -0.01%     
==========================================
  Files         842      842              
  Lines       18244    18245       +1     
==========================================
  Hits        16202    16202              
- Misses       2042     2043       +1     

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[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	v4.5

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