### 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_frame != nullptr) {
		*r_frame = nullptr;
	}
	if (r_line != nullptr) {
		*r_line = 0;
	}
	Item *item = p_item;
	while (item) {
		if (item->parent != nullptr && item->parent->type == ITEM_FRAME) {
			if (r_frame != nullptr) {
				*r_frame = static_cast<ItemFrame *>(item->parent);
			}
			if (r_line != nullptr) {
				*r_line = item->line;
			}
			return;
		}
		item = item->parent;
	}
}
RichTextLabel::Item *RichTextLabel::_find_indentable(Item *p_item) {