Check if blob file exists before deleting it from fs

[adzialocha]

Closes: https://github.com/p2panda/aquadoggo/issues/629

📋 Checklist

• Add tests that cover your changes
• [x] Add this PR to the Unreleased section in CHANGELOG.md
• [x] Link this PR to any issues it closes
• [x] New files contain a SPDX license header

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.84%. Comparing base (086d424) to head (ee67fde). Report is 16 commits behind head on main.

Files	Patch %	Lines
...doggo/src/materializer/tasks/garbage_collection.rs	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #636 +/- ## ========================================== - Coverage 92.75% 91.84% -0.91% ========================================== Files 104 104 Lines 20191 20617 +426 ========================================== + Hits 18728 18936 +208 - Misses 1463 1681 +218 ```

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