Fix validation when the JWE does not contains a JWS

[ycrumeyrolle]

Fix #504

[codecov]

Codecov Report

Merging #505 into master will increase coverage by 0.01%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
+ Coverage   63.11%   63.12%   +0.01%     
==========================================
  Files         165      165              
  Lines        8798     8806       +8     
  Branches     1449     1452       +3     
==========================================
+ Hits         5553     5559       +6     
- Misses       2727     2728       +1     
- Partials      518      519       +1     

Impacted Files	Coverage Δ
src/JsonWebToken/TokenValidationPolicyBuilder.cs	48.21% <50.00%> (-1.32%)	⬇️
src/JsonWebToken/SignatureValidationPolicy.cs	82.50% <66.66%> (-1.29%)	⬇️
src/JsonWebToken/JwtReader.cs	78.21% <100.00%> (+0.77%)	⬆️
src/JsonWebToken/TokenValidationPolicy.cs	92.64% <100.00%> (+1.47%)	⬆️

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