Fix "Attach Node Script" dialog being off-center on first popup

Created by: ghost

I noticed that creating a script from within the Script editor (File > New Script...) would have its popup be centered correctly, so I looked at its code and saw that its call to popup_centered gave it size information: https://github.com/godotengine/godot/blob/dd2cd06165670bb0d78bf4aa397935be15716e76/editor/plugins/script_editor_plugin.cpp#L971

I just did the same for the Scene dock's popup call.

Fixes #14659 (closed)