Update & fix C# blocks under tutorials/scripting/

- `_Process` and `_PhysicsProcess` take their delta argument as a `double`
- Use string interpolation instead of concatenating strings
- Use `string` instead of `String` (`using System` was removed from everywhere in a previous PR)
- Use `System.Array.Empty<T>()` to init empty arrays
- Use `SignalName.*` instead of `nameof(*)` for signals
- Do not use `event` as an argument name (that's a keyword)
- Match node paths to screenshots when retrieving nodes from the tree
- Add a C# example for scene unique nodes
- Update the "Cross language scripting" page
- Add a note about using `is` against `null`

tutorials/scripting/c_sharp/c_sharp_features.rst

@@ -67,7 +67,7 @@ Generic methods are also provided to make this type conversion transparent.
 
 To check if the node can be cast to Sprite2D, you can use the ``is`` operator.
 The ``is`` operator returns false if the node cannot be cast to Sprite2D,
-otherwise it returns true.
+otherwise it returns true. Note that using the ``is`` operator against ``null`` is always going to be falsy.
 
 .. code-block:: csharp
 
@@ -76,6 +76,11 @@ otherwise it returns true.
         // Yup, it's a Sprite2D!
     }
 
+    if (null is Sprite2D)
+    {
+        // This block can never happen.
+    }
+
 For more advanced type checking, you can look into `Pattern Matching <https://docs.microsoft.com/en-us/dotnet/csharp/pattern-matching>`_.
 
 .. _doc_c_sharp_signals:
@@ -107,7 +112,7 @@ If you want to connect a signal in the editor, you need to (re)build the project
 
     public void MyCallbackWithArguments(string foo, int bar)
     {
-        GD.Print("My callback with: ", foo, " and ", bar, "!");
+        GD.Print($"My callback with: {foo} and {bar}!");
     }
 
     public void SomeFunction()
@@ -122,11 +127,11 @@ Emitting signals is done with the ``EmitSignal`` method.
 
     public void SomeFunction()
     {
-        EmitSignal(nameof(MySignal));
-        EmitSignal(nameof(MySignalWithArguments), "hello there", 28);
+        EmitSignal(SignalName.MySignal);
+        EmitSignal(SignalName.MySignalWithArguments, "hello there", 28);
     }
 
-Notice that you can always reference a signal name with the ``nameof`` keyword (applied on the delegate itself).
+Notice that you can always reference a signal name with its generated ``SignalName``.
 
 It is possible to bind values when establishing a connection by passing a Godot array.