fix: Prevent panic when exp claim is empty or a string

[appleboy]

fix #192

[codecov-io]

Codecov Report

Merging #193 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   96.78%   96.85%   +0.06%     
==========================================
  Files           1        1              
  Lines         280      286       +6     
==========================================
+ Hits          271      277       +6     
  Misses          5        5              
  Partials        4        4

Impacted Files	Coverage Δ
auth_jwt.go	96.85% <100%> (+0.06%)	⬆️

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