More details for Include and then Include

It does not follow from the documentation what the behavior of `Include` and `ThenInclude` regarding `null`s of collection navigation properties. If included set of values is empty, will navigation property stay `null` or will it be initialized with empty collection? Worth adding the details to `Include`'s xmldoc.

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#### Document Details

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* ID: bbd36f35-e257-d0ed-8f28-f539ddcb71ec
* Version Independent ID: 093903d5-f7c7-5a78-c0e5-0e2662dcedef
* Content: [Loading Related Data - EF Core](https://docs.microsoft.com/en-us/ef/core/querying/related-data)
* Content Source: [entity-framework/core/querying/related-data.md](https://github.com/dotnet/EntityFramework.Docs/blob/master/entity-framework/core/querying/related-data.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @rowanmiller
* Microsoft Alias: **avickers**