fix: reject oauth logins when neither token references a user id

[FrenchGithubUser]

SYN-125

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.60%. Comparing base (31474b4) to head (71d480b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   84.58%   84.60%   +0.01%     
==========================================
  Files          13       13              
  Lines         902      903       +1     
  Branches      163      164       +1     
==========================================
+ Hits          763      764       +1     
  Misses         85       85              
  Partials       54       54              

Files with missing lines	Coverage Δ
synapse_token_authenticator/token_authenticator.py	71.65% <100.00%> (+0.22%)	⬆️

... and 1 file with indirect coverage changes

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[jason-famedly]

LGTM 🚀