### 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)

fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
			draw_box = true;
		}
		if (draw_box) {
			Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, fbg_index.y);
			for (int j = 0; j < sel.size(); j++) {
				Vector2 rect_off = line_off + Vector2(sel[j].x - hpad, -TS->shaped_text_get_ascent(p_rid) - vpad);
				Vector2 rect_size = Vector2(sel[j].y - sel[j].x + 2 * hpad, TS->shaped_text_get_size(p_rid).y + 2 * vpad);
				RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
			}
			fbg_index = Vector2i(end, start);
			draw_box = false;
		}
		if (change_color) {
			fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
		}
		last_color = color;
	}
	if (last_color.a > 0) {
		Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, end);
		for (int i = 0; i < sel.size(); i++) {
			Vector2 rect_off = line_off + Vector2(sel[i].x - hpad, -TS->shaped_text_get_ascent(p_rid) - vpad);
			Vector2 rect_size = Vector2(sel[i].y - sel[i].x + 2 * hpad, TS->shaped_text_get_size(p_rid).y + 2 * vpad);
			RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
		}
	}
}
Ref<RichTextEffect> RichTextLabel::_get_custom_effect_by_code(String p_bbcode_identifier) {