fix: omit empty help description section

[YNan-varamor]

Summary

Omits the default help Description: section when a command description is null, empty, or whitespace.

Why

Fixes #2114. Descriptions are optional, but the default help layout always wrote a description heading. That produced an empty section such as:

Description:

Changes

• Makes the default synopsis section return false when there is no description to render
• Adds a regression test for null, empty, and whitespace descriptions

Validation

• git diff --check

I attempted to run:

dotnet test src/System.CommandLine.Tests/System.CommandLine.Tests.csproj --filter "FullyQualifiedName~Synopsis_section_is_omitted_when_description_is_empty|FullyQualifiedName~Synopsis_section_properly_wraps_description"

but the local machine only has .NET SDK 10.0.203 and this repository's global.json currently requests 11.0.100-preview.4.26210.111, so the SDK could not be resolved.

AI assistance disclosure

This PR was prepared with assistance from OpenAI Codex. The change was reviewed locally and kept to a focused bug fix with a regression test.

[YNan-varamor]

@dotnet-policy-service agree

[jonsequitur]

This is a duplicate of #2814. Before submitting a PR, it's helpful to check the issue in question to see whether other PRs are linked to it.

[YNan-varamor]

Closing this because it duplicates #2814. Thanks for pointing that out.