Initialize editor values on first launch

[KoBeWi]

#74682 moved some defaults from constructor to clear() method. Normally clear() is called at startup, but there is only one case when it's not. When you open a fresh new project, which doesn't have any scene and opens an empty editor 🙃

Fixes #75749

[capnm]

It looped even in non-minimal project width scenes, the initialization seems to fix everything ...

[KoBeWi]

Opening any scene (even if it fails) will call reset(). So the only time when it could happen in non-empty project is when you don't have a default scene and disable scene restoring.

[capnm]

EDIT: Was also broken on every project that starts without a scene open + default settings.
(E.g. default scene, which is autoloaded calls a reset)

Now I see at project start (c151d32 + your PR)

ERROR: Condition "!instance" is true.
   at: instance_set_visible (servers/rendering/renderer_scene_cull.cpp:954)

EDIT: confirmed - the Error is coming from this PR.

[capnm]

RendererSceneCull::instance_set_visible(RendererSceneCull * const this, RID p_instance, bool p_visible) (servers/rendering/renderer_scene_cull.cpp:954)
	Instance *instance = instance_owner.get_or_null(p_instance);
	ERR_FAIL_COND(!instance);

RenderingServerDefault::instance_set_visible(RenderingServerDefault * const this, RID p1, bool p2) (servers/rendering/rendering_server_default.h:771)
Node3DEditor::clear(Node3DEditor * const this) (editor/plugins/node_3d_editor_plugin.cpp:7869)
Node3DEditor::Node3DEditor(Node3DEditor * const this) (editor/plugins/node_3d_editor_plugin.cpp:8657)
Node3DEditorPlugin::Node3DEditorPlugin(Node3DEditorPlugin * const this) (editor/plugins/node_3d_editor_plugin.cpp:8781)
EditorNode::EditorNode(EditorNode * const this) (editor/editor_node.cpp:7884)
Main::start() (main/main.cpp:2811)
main(int argc, char ** argv) (platform/linuxbsd/godot_linuxbsd.cpp:71)

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.