### 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)

List<Item *>::Element *E = p_from;
	while (E != nullptr) {
		ERR_CONTINUE(E->get()->type != ITEM_FRAME); 
		ItemFrame *frame = static_cast<ItemFrame *>(E->get());
		if (p_reverse_search) {
			for (int i = (int)frame->lines.size() - 1; i >= 0; i--) {
				if (_search_line(frame, i, p_string, -1, p_reverse_search)) {
					return true;
				}
			}
		} else {
			for (int i = 0; i < (int)frame->lines.size(); i++) {
				if (_search_line(frame, i, p_string, 0, p_reverse_search)) {
					return true;
				}
			}
		}
		E = p_reverse_search ? E->prev() : E->next();
	}
	return false;
}
bool RichTextLabel::_search_line(ItemFrame *p_frame, int p_line, const String &p_string, int p_char_idx, bool p_reverse_search) {