### 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 (r_click_item) {
		*r_click_item = nullptr;
	}
	if (r_click_char != nullptr) {
		*r_click_char = 0;
	}
	if (r_outside != nullptr) {
		*r_outside = true;
	}
	Size2 size = get_size();
	Rect2 text_rect = _get_text_rect();
	int vofs = vscroll->get_value();
	
	int to_line = main->first_invalid_line.load();
	int from_line = _find_first_line(0, to_line, vofs);
	Point2 ofs = text_rect.get_position() + Vector2(0, main->lines[from_line].offset.y - vofs);
	while (ofs.y < size.height && from_line < to_line) {
		MutexLock lock(main->lines[from_line].text_buf->get_mutex());
		_find_click_in_line(p_frame, from_line, ofs, text_rect.size.x, p_click, r_click_frame, r_click_line, r_click_item, r_click_char, false, p_meta);
		ofs.y += main->lines[from_line].text_buf->get_size().y + main->lines[from_line].text_buf->get_line_count() * theme_cache.line_separation;
		if (((r_click_item != nullptr) && ((*r_click_item) != nullptr)) || ((r_click_frame != nullptr) && ((*r_click_frame) != nullptr))) {
			if (r_outside != nullptr) {
				*r_outside = false;
			}
			return;
		}
		from_line++;
	}
}
float RichTextLabel::_find_click_in_line(ItemFrame *p_frame, int p_line, const Vector2 &p_ofs, int p_width, const Point2i &p_click, ItemFrame **r_click_frame, int *r_click_line, Item **r_click_item, int *r_click_char, bool p_table, bool p_meta) {