Sdk attribute on project causes import to fail

Importing a project that has the Sdk attribute causes Import to fail.

**Repro:**
```xml

```
```xml

```

Run `dotnet msbuild main.proj`

**Error:**
> C:\dev\dotnet\sdk\1.0.0-preview4-004215\Sdks\Microsoft.NET.Sdk\Sdk\Sdk.targets(35,3): error MSB4102: The value "" of the "Project" attribute in element is invalid. Parameter "path" cannot have zero length. [C:\dev\proj\main.proj]