Proposal: [WinUI-BuildSampleApps] Optimize pipeline conditions and add error handling in NuGet source script

### Title

[WinUI-BuildSampleApps] Optimize pipeline conditions and add error handling in NuGet source script

### Summary

Proposing an improvement to `WinUI-BuildSampleApps-Steps.yml` to enhance the robustness of the NuGet configuration PowerShell script and streamline redundant pipeline condition checks.

### Rationale

- Enhances CI/CD pipeline reliability and maintainability for sample app builds.
- Streamlines repetitive condition syntax to keep pipeline templates clean and robust.

### Scope

- Must: Optimize script clarity and configuration handling within WinUI-BuildSampleApps-Steps.yml.
- Won't: Alter core application build logic or functional output.

### Important Notes

- Tested via Azure Pipelines template syntax conventions.

### Open Questions

- None at this stage.

Start with WinUI-BuildSampleApps-Steps.yml and inspect the NuGet configuration PowerShell script and its pipeline conditions. Validate the template using Azure Pipelines syntax conventions; done means the conditions are less repetitive and the script has clearer configuration handling and error handling without changing core build logic or output.

Written by the indexing model from the issue text.