feat: add asset existence check before replacing static URLs

[bra-i-am]

Description

This merge request solves an issue where the save warning is shown without making changes to new courses. Additionally, it also adds an existence check before replacing the static URLs

Supporting information

#1265

Testing instructions

1. Mount this MFE in a Tutor environment and git switch to this branch
2. Enable contentstore.new_studio_mfe.use_new_schedule_details_page
3. Create course in Studio
4. Open Settings -> Schedule & details
5. Once you enter this page, you shouldn't watch the warning popover to jump out as you just made a change
6. Additionally, in the Course overview it must be displayed the default images if the assets are not found

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.73%. Comparing base (06d591d) to head (0123b77).
Report is 84 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1708      +/-   ##
==========================================
+ Coverage   93.37%   93.73%   +0.36%     
==========================================
  Files        1110     1143      +33     
  Lines       22244    23792    +1548     
  Branches     4715     5021     +306     
==========================================
+ Hits        20770    22302    +1532     
- Misses       1409     1422      +13     
- Partials       65       68       +3     

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