### 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 = brk_pos > pos ? p_bbcode.substr(pos, brk_pos - pos) : "";
		
		if (after_list_close_tag || after_list_open_tag) {
			txt = txt.trim_prefix("\n");
		}
		if (brk_pos == p_bbcode.length()) {
			
			if (txt.is_empty() && after_list_open_tag) {
				txt = "\n";
			}
			if (!txt.is_empty()) {
				add_text(txt);
			}
			break; 
		}
		int brk_end = p_bbcode.find("]", brk_pos + 1);
		if (brk_end == -1) {
			
			txt += p_bbcode.substr(brk_pos, p_bbcode.length() - brk_pos);
			add_text(txt);
			break;
		}
		String tag = p_bbcode.substr(brk_pos + 1, brk_end - brk_pos - 1);
		Vector<String> split_tag_block = tag.split(" ", false);
		
		String bbcode_name;
		typedef HashMap<String, String> OptionMap;
		OptionMap bbcode_options;
		if (!split_tag_block.is_empty()) {