fix: do not update refresh token on session refresh, if empty

[jkroepke]

What this PR does / why we need it

Fixes #338

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

• fixes #

Special notes for your reviewer

Particularly user-facing changes

Checklist

Complete these before marking the PR as ready to review:

• DCO signed
• The PR title has a summary of the changes
• The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 76.41509% with 50 lines in your changes missing coverage. Please review.

Project coverage is 81.80%. Comparing base (1dc2a22) to head (c0de6ee).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/oauth2/handler.go	40.62%	14 Missing and 5 partials ⚠️
internal/oauth2/refresh.go	32.00%	15 Missing and 2 partials ⚠️
internal/openvpn/callbacks.go	64.70%	6 Missing ⚠️
internal/storage/storage.go	58.33%	4 Missing and 1 partial ⚠️
internal/openvpn/client.go	97.01%	2 Missing ⚠️
internal/oauth2/providers/generic/oidc.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
- Coverage   82.50%   81.80%   -0.70%     
==========================================
  Files          50       51       +1     
  Lines        3081     3150      +69     
==========================================
+ Hits         2542     2577      +35     
- Misses        408      437      +29     
- Partials      131      136       +5     

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