fix: incorrect initialization for nullable nested memberpath unflattening

[TimothyMakkison]

Fix incorrect initialzation code for nested nullable members when unflattening

Description

When using MapProperty to map to deeply nested nullable members, Mapperly will generate a series of identical statements initializing the same field. This is because all the MemberNullAssignmentInitializerMapping mistakenly use the same IReadOnlyCollection so will all update the same path.

Added a ToArray to ObjectPathNullableSubPaths to collect the paths into separate collections.

Example bug

var target = new global::B();
target.Value.Nested ??= new();
target.Value.Nested ??= new();
target.Value.Nested ??= new();
target.Value.Nested.Id = source.MyValueId;
target.Value.Nested.Id2 = source.MyValueId2;
return target;

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Unit tests are added/updated

[codecov]

Codecov Report

Merging #591 (746c793) into main (208e8b4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #591   +/-   ##
=======================================
  Coverage   90.59%   90.59%           
=======================================
  Files         173      173           
  Lines        5921     5921           
  Branches      757      757           
=======================================
  Hits         5364     5364           
  Misses        397      397           
  Partials      160      160           

Files Changed	Coverage Δ
src/Riok.Mapperly/Symbols/MemberPath.cs	83.13% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 2.9.0-next.4 🎉

The release is available on:

• GitHub release
• v2.9.0-next.4

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.0.0 🎉

The release is available on:

• GitHub release
• v3.0.0

Your semantic-release bot 📦🚀