### 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)

#include "rich_text_label.h"
#include "core/input/input_map.h"
#include "core/math/math_defs.h"
#include "core/os/keyboard.h"
#include "core/os/os.h"
#include "core/string/translation.h"
#include "scene/gui/label.h"
#include "scene/resources/atlas_texture.h"
#include "scene/scene_string_names.h"
#include "servers/display_server.h"
#include "modules/modules_enabled.gen.h" 
#ifdef MODULE_REGEX_ENABLED
#include "modules/regex/regex.h"
#endif
RichTextLabel::Item *RichTextLabel::_get_next_item(Item *p_item, bool p_free) const {