### 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)

ERR_FAIL_COND_V(p_frame == nullptr, false);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), false);
	Line &l = p_frame->lines[p_line];
	String txt;
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		switch (it->type) {
			case ITEM_NEWLINE: {
				txt += "\n";
			} break;
			case ITEM_TEXT: {
				ItemText *t = static_cast<ItemText *>(it);
				txt += t->text;
			} break;
			case ITEM_IMAGE: {
				txt += " ";
			} break;
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);