Improved codegen in ImageSharp.Guard

[Sergio0694]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Small follow-up for SixLabors/SharedInfrastructure#12 for the ImageSharp-specific Guard API.

[codecov]

Codecov Report

Merging #1333 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   82.77%   82.77%           
=======================================
  Files         690      690           
  Lines       30973    30973           
  Branches     3511     3511           
=======================================
  Hits        25639    25639           
  Misses       4613     4613           
  Partials      721      721           

Flag	Coverage Δ
#unittests	82.77% <ø> (ø)

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