Add support for unencrypted private crypt4gh keys

[pontus]

This fixes support for unencrypted private crypt4gh keys. Support essentially existed but due to a logic error wasn't usable (KDF was checked against a list of actual KDFs, not including none, which is used for the unencrypted case) and failed if no match was found.

This moves the unencrypted case up a bit and lets the following code be a little simpler, also adds a testcase for an unencrypted key.

[blankdots]

i think we can go with version 1.9.1 as the way i read it it is a bug fix
change version https://github.com/neicnordic/crypt4gh/blob/master/internal/version/version.go#L10

[blankdots]

how about we also add a test for decrypting with unprotected key, just in case

[dbampalikis]

Apart for the test mentioned by Stefan, it looks good. Also tested it with the pipeline

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 69.75%. Comparing base (88c2108) to head (9060c4f).
Report is 4 commits behind head on master.

Files	Patch %	Lines
keys/keys.go	60.00%	5 Missing and 5 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   69.72%   69.75%   +0.03%     
==========================================
  Files           6        6              
  Lines         862      873      +11     
==========================================
+ Hits          601      609       +8     
  Misses        141      141              
- Partials      120      123       +3     

Flag	Coverage Δ
unittests	69.75% <60.00%> (+0.03%)	⬆️

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