Only override default config values if config file exists and is valid

[johannaengland]

Scope and purpose

Fixes something overlooked in #315. In case no config file is given and none exists under the default name the function load_config returns None, which overrides the default values that state.config has before.

Contributor Checklist

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• Added a changelog fragment for towncrier
  • the oversight has not made it into a release yet
• Added/amended tests for new/changed code
  • but manually tested
• Added/changed documentation
• Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.63%. Comparing base (3078e55) to head (67323c9).
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   98.60%   98.63%   +0.03%     
==========================================
  Files          77       77              
  Lines        9688     9690       +2     
==========================================
+ Hits         9552     9557       +5     
+ Misses        136      133       -3     

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