### 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)

Item *item = p_item;
	float margin = 0.0;
	while (item) {
		if (item->type == ITEM_INDENT) {
			Ref<Font> font = p_base_font;
			int font_size = p_base_font_size;
			ItemFont *font_it = _find_font(item);
			if (font_it) {
				if (font_it->font.is_valid()) {
					font = font_it->font;
				}
				if (font_it->font_size > 0) {
					font_size = font_it->font_size;