NullReference Nested resource include chains

[Maan511]

I found a bug when including nested reources. Consider the following example:

class Book {
  // This is optional
  [HasOne("Author")]
  public virtual Author Author { get; set; }
}

class Author {
  [HasMany("Image")]
  public virtual IList<Image> Images { get; set; }
}

class Image {
}

Now when requesting Books with ?Include="Author.Images" An exception is thrown from the ResourceGraph, when a book doesn't have an author. Because the parent entity is null.

JsonApiDotNetCore/src/JsonApiDotNetCore/Internal/ResourceGraph.cs

Lines 118 to 120 in 8570b21

	public object GetRelationship<TParent>(TParent entity, string relationshipName)
	{
	var parentEntityType = entity.GetType();

[milosloub]

I have the same issue