### 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)

String txt;
	ERR_FAIL_COND_V(p_frame == nullptr, txt);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), txt);
	Line &l = p_frame->lines[p_line];
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	int end_idx = 0;
	if (it_to != nullptr) {
		end_idx = it_to->index;
	} else {
		for (Item *it = l.from; it; it = _get_next_item(it)) {
			end_idx = it->index + 1;
		}
	}
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		if (it->type == ITEM_TABLE) {
			ItemTable *table = static_cast<ItemTable *>(it);
			for (Item *E : table->subitems) {
				ERR_CONTINUE(E->type != ITEM_FRAME); 
				ItemFrame *frame = static_cast<ItemFrame *>(E);
				for (int i = 0; i < (int)frame->lines.size(); i++) {
					txt += _get_line_text(frame, i, p_selection);
				}
			}
		}