fix: do not store refresh token, if empty.

[jkroepke]

What this PR does / why we need it

#338 (comment)

Which issue this PR fixes

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

• fixes Missing required parameter: refresh_token #338

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 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.17%. Comparing base (1b04674) to head (231a407).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/oauth2/refresh.go	61.53%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #349      +/-   ##
==========================================
+ Coverage   81.82%   82.17%   +0.35%     
==========================================
  Files          51       51              
  Lines        3152     3153       +1     
==========================================
+ Hits         2579     2591      +12     
+ Misses        437      428       -9     
+ Partials      136      134       -2     

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