Fix provideRunner with no managers enabled.

[mpass99]

If only AWS was enabled, a 500 error instead of a 404 error was returned. If both Nomad and AWS was disabled, the requests returned with a 200 response but an empty body. A silent nil-dereference was triggered.

Closes https://github.com/orgs/openHPI/projects/4/views/1?pane=issue&itemId=81052193

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.40%. Comparing base (5b64aee) to head (11b18aa).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/environment/abstract_manager.go	80.00%	1 Missing ⚠️
internal/runner/nomad_manager.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #704      +/-   ##
==========================================
+ Coverage   76.03%   76.40%   +0.37%     
==========================================
  Files          43       43              
  Lines        3676     3688      +12     
==========================================
+ Hits         2795     2818      +23     
+ Misses        645      635      -10     
+ Partials      236      235       -1     

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[MrSerth]

Nice, thank you! I tested the functionality and was able to confirm that it now works as expected.

As part of my review process, I also observed that synchronizing any environment to Poseidon was successful when no runner manager was enabled. Hence, I amended your PR to change this behavior and further fix a superfluous header write (due to a missing return statement). With that, I am happy to merge this PR 👍.