Issue with Bamboos git-checkout and Source Link

Hi,

We use a self-hosted Bitbucket instance as git host, and build our packages with Bamboo.

Bamboo has a very special way of checking out/cloning git repositories in a CI build. See https://jira.atlassian.com/browse/BAM-15012 for details, but the short version is, that for local git checkouts it sets the origin-url to either a local path, or even to `file://nothing`.
So when trying to read that automatically you will end up with an error like:
`The value of SourceRoot.RepositoryUrl with identity '' is invalid: 'file://nothing/` what causes the build to fail.

The question now is, which configuration is needed, so that Source Link won't produce errors or warnings?
Currently we go with:

```xml

https://our.bitbucket.instance/scm//.git

```

The generated NuGet package metadata contains the repository URL as referenced, but the build produces the following two warnings for every package that configures Source Link information that way:

```
/packages/microsoft.build.tasks.git/1.0.0/build/Microsoft.Build.Tasks.Git.targets(24,5): warning : Repository '\\nothing' has no remote. [//src//.csproj]
/packages/microsoft.sourcelink.common/1.0.0/build/Microsoft.SourceLink.Common.targets(52,5): warning : Source control information is not available - the generated source link is empty. [//src//.csproj]
```

Additional questions are:
* Is there a configuration that would get rid of those warnings? I thought explicitly specifying the `RepositoryUrl` property would stop Source Link from trying to look up repository information.
* Will Source Link work regardless of those warnings? As said, there *is* the Repository URL getting written in the Package metadata

Thanks for help,
Peter