Fix PackageRelationship.GetHashCode Exception because the source can be null

[GGJason]

The source can be null. I was stuck by the exception when I want to copy a table row.
To fix the problem, I add a null-condition when getting the hashcode of source.And anohter null-coalescing operator to return 0 when the hashcode result is null.

public override int GetHashCode()
{
    return this.id.GetHashCode() + this.relationshipType.GetHashCode()
        + this.source?.GetHashCode() ?? 0 + this.targetMode.GetHashCode()
        + this.targetUri.GetHashCode();
}

[tonyqus]

LGTM