### 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)

Vector2 off;
	bool line_clicked = false;
	float text_rect_begin = 0.0;
	int char_pos = -1;
	Line &l = p_frame->lines[p_line];
	MutexLock lock(l.text_buf->get_mutex());
	bool rtl = (l.text_buf->get_direction() == TextServer::DIRECTION_RTL);
	bool lrtl = is_layout_rtl();
	
	bool table_hit = false;
	Vector2i table_range;
	float table_offy = 0.f;
	ItemFrame *table_click_frame = nullptr;
	int table_click_line = -1;
	Item *table_click_item = nullptr;
	int table_click_char = -1;
	for (int line = 0; line < l.text_buf->get_line_count(); line++) {
		RID rid = l.text_buf->get_line_rid(line);
		float width = l.text_buf->get_width();
		float length = TS->shaped_text_get_width(rid);
		if (rtl) {
			off.x = p_width - l.offset.x - width;
			if (!lrtl && p_frame == main) { 
				off.x -= scroll_w;
			}
		} else {
			off.x = l.offset.x;
			if (lrtl && p_frame == main) { 
				off.x += scroll_w;