fix #1031

[dogdie233]

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What cause the issue

In line 94

https://github.com/aaubry/YamlDotNet/blob/b8ac2a98ffcc12434eff6c6abb75b38ad1b1ab04/YamlDotNet.Analyzers.StaticGenerator/TypeFactoryGenerator.cs#L92-L98

When the StaticContext to be generated is in the global namespace, syntaxReceiver.YamlStaticContextType?.GetNamespace() returns an empty string instead of null. As a result, the ?? operator does not take effect.

So, the generated code in YamlDotNetAutoGraph.g.cs ends up as follows:

...
namespace 
{
    ...

Since there is no identifier following namespace, the compilation fails.

About unit test case

Due to the stringent conditions required to trigger this issue, I am unsure how to write a unit test. Should I create a new console project to reproduce the issue?

A potential problem

When the auto-generated StaticContext is in the global namespace, other classes, such as StaticObjectFactory, will also be created in the global namespace. This could potentially conflict with other classes.