Check if refresh token is undefined and do not store it in this case

[tippexs]

Proposed changes

In some cases the token response from the Identity Provider (IdP) does not contain a refresh token. In this case the variable tokenset.refresh_token will be undefined. As undefined is unequal to the current refresh-token undefined is stored as the tokens value in the key-value store. This results in a deletion of the token from the key-value-store.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [X ] I have read the CONTRIBUTING doc
• [X ] I have added tests that prove my fix is effective or that my feature works
• [ X] I have checked that all unit tests pass after adding my changes
• [ X] I have updated necessary documentation
• [ X ] I have rebased my branch onto master
• [ X] I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[codecov-commenter]

Codecov Report

Merging #2260 (fd70a12) into master (6b6c675) will not change coverage.
The diff coverage is n/a.

❗ Current head fd70a12 differs from pull request most recent head 6b54acf. Consider uploading reports for the commit 6b54acf to get more accurate results

@@           Coverage Diff           @@
##           master    #2260   +/-   ##
=======================================
  Coverage   53.39%   53.39%           
=======================================
  Files          43       43           
  Lines       13413    13413           
=======================================
  Hits         7162     7162           
  Misses       6022     6022           
  Partials      229      229           

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[pleshakov]

@lucacome @tippexs is it a bug fix? shall we tag it as a bug fix?

[lucacome]

yeah I think it makes sense to add the bug label