### 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)

txt += "[" + tag;
				add_text(txt);
				after_list_open_tag = false;
				after_list_close_tag = false;
				pos = brk_end;
				continue;
			}
			if (txt.is_empty() && after_list_open_tag) {
				txt = "\n"; 
			}
			after_list_open_tag = false;
			if (tag == "/ol" || tag == "/ul") {
				if (!txt.is_empty()) {
					
					
					if (!txt.ends_with("\n")) {
						txt += "\n";
					}
				} else if (!after_list_close_tag) {
					txt = "\n"; 
				}
				after_list_close_tag = true;
			} else {
				after_list_close_tag = false;
			}
			if (!txt.is_empty()) {
				add_text(txt);
			}
			tag_stack.pop_front();
			pos = brk_end + 1;