Command-line build fails for DNX projects when global.json is empty.

[mpiroc]

When a vanilla DNX project is created, a global.json file containing the sdk.version property is generated. When a Service Fabric DNX project is created, an empty global.json file is generated (see #1654).

If global.json is missing the sdk.version property, command-line builds fail. This is because the GetRuntimeToolingPath task ($/DevDiv/Offcycle/WPT/WebToolsExtensions/Main/src/ProjectK/MSBuildTasks/GetRuntimeToolingPath) throws an exception if sdk.version is missing from global.json. This task is invoked by C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\DNX\Microsoft.DNX.targets.

There are two approaches we could take to fix this:

1. Make sure global.json is generated with an sdk.version property (it should probably be beta4 for now).
2. Modify the build targets to use a default value (latest?) if the sdk.version property does not exist.

The issue if we only take the second approach is that Service Fabric doesn't currently support latest (beta5). So if we generate an empty global.json, and the msbuild targets default to latest, it will break Service Fabric customers. So ideally, we would implement both approaches.

[muratg]

@mayurid @PradeepKadubandi this should probably go into Tooling repo.