Add special notes for null C# Variant in several pages

[GoForceX]

Your Godot version:

v4.4.1.stable.mono.official [49a5bc7b6]

Issue description:

Some function documentation entries for returning a Variant mention the special return value of null. However, in C#, returning null directly is not feasible since Variant is a struct. This creates challenges for developers in identifying the correct approach to handle a null Variant, often requiring them to search through multiple documents for a solution. It is counter-intuitive to tell devs to return null when it is not actually possible to return null.

It might be caused by C# / GDScript inconsistencies, and it might be better to add some notes on these function docs to mention return default or return new Godot.Variant().

URL to the documentation page (if already existing):

https://docs.godotengine.org/en/latest/classes/class_control.html#class-control-private-method-get-drag-data

... Returns null if there is no data to drag ...

... and other doc pages that has the same issue.

[AThousandShips]

This is documented in the C# variant docs which is expected reading for how to handle that difference, I'm not sure we should clutter the class reference with this, the same with other quirks with C#

[GoForceX]

This is documented in the [C# variant docs]

Thanks for the reply. Just find it a little bit difficult to find the doc regarding of the issue. One using C# may also encounter the null problem and they might have to browse through multiple doc pages to get the explanation of it.

(Could be through Variant -> C# API differences to GDScript -> C# Variant)

Alternatively, it could be more helpful to include the explanation for the null C# Variant directly in the Variant documentation page. However, doing so might clutter the documentation and reduce its clarity. Not really sure about doing that.

[AThousandShips]

It says:

Since the Variant type in C# is a struct, it can't be null. To create a "null" Variant, use the default keyword or the Godot.Variant parameterless constructor.