fix: Allow refreshing of the token when we don't have a user

[NiallBunting]

Description

This doesn't fix the user being logged out (#47) but does allow updating of a refresh token without already having a user logged in.

This allows my work around to work without having to have a custom version of this library.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[ahmednfwela]

LGTM, thanks for your contribution

[ahmednfwela]

can you please rename the pr to start with "fix: "

[NiallBunting]

Updated :)

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (8c068e2) 32.38% compared to head (f05929e) 15.02%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
packages/oidc/lib/src/managers/user_manager.dart	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #48       +/-   ##
===========================================
- Coverage   32.38%   15.02%   -17.36%     
===========================================
  Files          43        4       -39     
  Lines        1260      406      -854     
===========================================
- Hits          408       61      -347     
+ Misses        852      345      -507     

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