fix: prevent generation of empty if not null statements

[TimothyMakkison]

Prevent mapperly from generating empty if not null statements

Description

Modify NullDelegateExistingTargetMapping to check if the body contains any elements, not generating the if statement when empty.

Fixes #497

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[codecov]

Codecov Report

Merging #576 (2ffcc7e) into main (b238481) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #576   +/-   ##
=======================================
  Coverage   91.08%   91.09%           
=======================================
  Files         167      167           
  Lines        5656     5660    +4     
  Branches      716      717    +1     
=======================================
+ Hits         5152     5156    +4     
  Misses        351      351           
  Partials      153      153           

Impacted Files	Coverage Δ
...xistingTarget/NullDelegateExistingTargetMapping.cs	85.71% <100.00%> (+5.71%)	⬆️

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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

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[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 📦🚀