TextEdit: prevent the copy of an empty string
Created by: lupoDharkael
With this change the clipboard isn't deleted after you hit Ctrl+C on an empty line. Note: RichTextLabel checks for empty strings before the copy to clipboard: https://github.com/godotengine/godot/blob/4cf5bb027678717263476e676cd23f881eef1ca1/scene/gui/rich_text_label.cpp#L2074