Switch with no value uses default

### Issue Description
Switches infer default value when not provided.

### Steps to Reproduce
```
> dotnet build /nodemode
Microsoft (R) Build Engine version 17.0.0+c9eb9dd64 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

MSBUILD : error MSB1033: Node number is not valid. .
Switch: 0

For switch syntax, type "MSBuild -help"
```

### Expected Behavior
It should not tell me I put in a 0 when I didn't.

### Analysis
This looks like a default value problem, but I'm not sure what the best fix is. Having `Switch: ` is a bad experience, too.

Reproduce the behavior with `dotnet build /nodemode` and inspect the command-line parsing path for the `/nodemode` switch. The payload names no source file or test, so first locate the relevant parser and existing switch-validation tests; done means the error no longer reports an inferred `0` when no value was supplied.

Written by the indexing model from the issue text.