Additional Files with an empty target path will crash the source generator

[chsienki]

The razor source generator requires every .cshtml or .razor file to have an associated TargetPath metadata item. We report a diagnostic to say that a given item didn't have a target path (https://github.com/dotnet/sdk/blob/4ac98544c9b8e1a1e433675b9bd3290c0d0578d5/src/RazorSdk/SourceGenerators/RazorSourceGenerator.RazorProviders.cs#L73) but try and continue when it is empty.

We should warn and stop processing when the target path is either null or empty.

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The actual crash happens because

• RazorSourceGenerator.ComputeProjectItems finds an empty value, not a null so continues
  https://github.com/dotnet/sdk/blob/4ac98544c9b8e1a1e433675b9bd3290c0d0578d5/src/RazorSdk/SourceGenerators/RazorSourceGenerator.RazorProviders.cs#L65
• SourceGeneratorProjectItem.RelativePhysicalPath receives an empty value https://github.com/dotnet/sdk/blob/04b2d8c35736410847ae8c8d8035abfc61398a8c/src/RazorSdk/SourceGenerators/SourceGeneratorProjectItem.cs#L19
• DefaultMetadataIdentifierFeature.GetIdentifier returns null due to the relative path being null or empty https://github.com/dotnet/razor-compiler/blob/a4df79c2bcf7d6e79cf870f84cf9fe3170c83fcf/src/Microsoft.AspNetCore.Razor.Language/src/Extensions/DefaultMetadataIdentifierFeature.cs#L28
• CreateNewOnMetadataUpdateAttributePass creates an null valued RazorCompiledItemMetadataAttributeIntermediateNode
  https://github.com/dotnet/razor-compiler/blob/77912f7f843df05dc8c106a87188985834a0b632/src/Microsoft.AspNetCore.Mvc.Razor.Extensions/src/CreateNewOnMetadataUpdateAttributePass.cs#L48
• During file writing MetadataAttributeTargetExtension passes the null valued Value to the CodeWriter.WriteStringLiteral which throws.
  https://github.com/dotnet/razor-compiler/blob/77912f7f843df05dc8c106a87188985834a0b632/src/Microsoft.AspNetCore.Razor.Language/src/Extensions/MetadataAttributeTargetExtension.cs#L62