Remove redundant code

[TimothyMakkison]

Minor code cleanup

• Removed redundant sc_clamp overload
• Removed redundant Update overload
• Spans aren't nullable so changed the null check to a default check.
• Updated test

[codecov-commenter]

Codecov Report

Merging #82 (8e1e25c) into master (ff8f017) will increase coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   82.39%   82.53%   +0.14%     
==========================================
  Files         104      104              
  Lines        5293     5285       -8     
  Branches      329      328       -1     
==========================================
+ Hits         4361     4362       +1     
+ Misses        807      800       -7     
+ Partials      125      123       -2     

Impacted Files	Coverage Δ
...eto/Cryptography/Internal/Ed25519Ref10/sc_clamp.cs	100.00% <ø> (+50.00%)	:arrow_up:
src/Paseto/Cryptography/Sha512.cs	88.40% <100.00%> (+6.21%)	:arrow_up:

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