Return empty path when outline width is less than or equal to zero

[tocsoft]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
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• [x] I have provided test coverage for my change (where applicable)

Description

Updates the GenerateOutline extensions so that when an invalid width (<= 0) is specified that instead of looping it returns an EmptyPath (a new type to represent an empty path without addition allocations).

This resolves #224

[codecov[bot]]

Codecov Report

Merging #225 (9e2259d) into main (764f6f1) will increase coverage by 0%. The diff coverage is 61%.

@@         Coverage Diff         @@
##           main   #225   +/-   ##
===================================
  Coverage    71%    71%           
===================================
  Files        87     88    +1     
  Lines      5349   5362   +13     
  Branches   1094   1096    +2     
===================================
+ Hits       3835   3845   +10     
- Misses     1295   1298    +3     
  Partials    219    219           

Flag	Coverage Δ
unittests	71% <61%> (+<1%)	:arrow_up:

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Impacted Files	Coverage Δ
src/ImageSharp.Drawing/Shapes/EmptyPath.cs	37% <37%> (ø)
...ImageSharp.Drawing/Shapes/OutlinePathExtensions.cs	100% <100%> (+3%)	:arrow_up:
src/ImageSharp.Drawing/Shapes/Path.cs	75% <100%> (+<1%)	:arrow_up:

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