fix(datastore): handle not found error for S3 HeadObject

[corrieriluca]

The Check operation on the datastore was not working as expected when using S3 (tested with MinIO):

• NotFound error was not correctly checked
• A nil-pointer dereference exception was raised on the checksum because S3 does not set a checksum by default in its responses.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.

Project coverage is 44.93%. Comparing base (125b9b0) to head (174927b).

Files with missing lines	Patch %	Lines
internal/datastore/storage/s3/s3.go	0.00%	17 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
- Coverage   44.98%   44.93%   -0.05%     
==========================================
  Files          79       79              
  Lines        5753     5759       +6     
==========================================
  Hits         2588     2588              
- Misses       2950     2956       +6     
  Partials      215      215              

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