### 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 (tag != "/img" && tag != "/dropcap") {
				pop();
			}
			continue;
		}
		if (tag == "ol" || tag.begins_with("ol ") || tag == "ul" || tag.begins_with("ul ")) {
			if (txt.is_empty() && after_list_open_tag) {
				txt = "\n"; 
			}
			after_list_open_tag = true;
		} else {
			after_list_open_tag = false;
		}
		if (!txt.is_empty()) {
			add_text(txt);
		}
		after_list_close_tag = false;
		if (tag == "b") {
			
			in_bold = true;
			if (in_italics) {
				_push_def_font(BOLD_ITALICS_FONT);
			} else {
				_push_def_font(BOLD_FONT);
			}
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag == "i") {
			
			in_italics = true;