### 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 *indentable = p_item;
	while (indentable) {
		if (indentable->type == ITEM_INDENT || indentable->type == ITEM_LIST) {
			return indentable;
		}
		indentable = indentable->parent;
	}
	return indentable;
}
RichTextLabel::ItemFont *RichTextLabel::_find_font(Item *p_item) {
	Item *fontitem = p_item;
	while (fontitem) {
		if (fontitem->type == ITEM_FONT) {
			ItemFont *fi = static_cast<ItemFont *>(fontitem);
			switch (fi->def_font) {
				case NORMAL_FONT: {
					if (fi->variation) {
						Ref<FontVariation> fc = fi->font;
						if (fc.is_valid()) {
							fc->set_base_font(theme_cache.normal_font);
						}
					} else {
						fi->font = theme_cache.normal_font;
					}
					if (fi->def_size) {
						fi->font_size = theme_cache.normal_font_size;
					}
				} break;
				case BOLD_FONT: {
					if (fi->variation) {