[Improvement] Enable nullable on GitVersion.Core

[Evangelink]

Is your improvement request related to a problem? Please describe.

Enable the nullable functionality on GitVersion.Core project.

Detailed Description

This will help to understand (and also help analyzers) when a piece of code is expected to be always not-null or could be null.

Context

See #2763

Possible Implementation

Enable nullable on the project and fix warnings.

[github-actions]

🎉 This issue has been resolved in version 5.6.11 🎉
The release is available on:

• GitHub Release
• Chocolatey Package
• .Net Global Tool

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