Add special notes for null C# Variant in several pages

**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.