GDScript: Don't fail when freed object is return

This is check is a bit too eager. The user should be able to handle the
return value even if it's a freed object.

modules/gdscript/tests/scripts/runtime/features/getter_with_freed_object.gd

@@ -0,0 +1,15 @@
+# https://github.com/godotengine/godot/issues/68184
+
+var node: Node:
+	get:
+		return node
+	set(n):
+		node = n
+
+
+func test():
+	node = Node.new()
+	node.free()
+
+	if !is_instance_valid(node):
+		print("It is freed")

modules/gdscript/tests/scripts/runtime/features/getter_with_freed_object.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+It is freed