Use ContentRootPath if WebRootPath is null to find the full path

[oyvindbergbouvet]

https://github.com/reactjs/React.NET/blob/6d5bdb03157798e661900f895729496c0ffd9570/src/React.AspNet.Middleware/AspNetFileSystem.cs#L44

Use ContentRootPath if WebRootPath is null

• if both are null, then throw

Some applications use react ssr, server react components through API's without a wwwroot folder, and some applications use the react ssr only, still no wwwroot.

Or an additional method allowing the consumer of the package to pass in full paths of the scripts.