### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

}
			if (k->is_action("ui_home", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(0);
				handled = true;
			}
			if (k->is_action("ui_end", true) && vscroll->is_visible_in_tree()) {
				vscroll->set_value(vscroll->get_max());
				handled = true;
			}
			if (is_shortcut_keys_enabled()) {
				if (k->is_action("ui_text_select_all", true)) {
					select_all();
					handled = true;
				}
				if (k->is_action("ui_copy", true)) {
					selection_copy();
					handled = true;
				}
			}
			if (k->is_action("ui_menu", true)) {
				if (context_menu_enabled) {
					_update_context_menu();
					menu->set_position(get_screen_position());
					menu->reset_size();
					menu->popup();
					menu->grab_focus();
				}
				handled = true;
			}
			if (handled) {