Allow copying empty directories with <Content>

This issue has been inspired by dotnet/cli#2911. In short, if I specify something like this:

```xml

Always

```

MSBuild will throw an error saying that the `Copy` task does not support copying directories. I can put in a glob, but if there are no files, nothing gets copied out (with a glob the *why* of that behavior is clear to me at least).

/cc @moozzyk