### 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)

st_parser_type = TextServer::STRUCTURED_TEXT_DEFAULT;
						} else if (subtag_a[1] == "u" || subtag_a[1] == "uri") {
							st_parser_type = TextServer::STRUCTURED_TEXT_URI;
						} else if (subtag_a[1] == "f" || subtag_a[1] == "file") {
							st_parser_type = TextServer::STRUCTURED_TEXT_FILE;
						} else if (subtag_a[1] == "e" || subtag_a[1] == "email") {
							st_parser_type = TextServer::STRUCTURED_TEXT_EMAIL;
						} else if (subtag_a[1] == "l" || subtag_a[1] == "list") {
							st_parser_type = TextServer::STRUCTURED_TEXT_LIST;
						} else if (subtag_a[1] == "n" || subtag_a[1] == "gdscript") {
							st_parser_type = TextServer::STRUCTURED_TEXT_GDSCRIPT;
						} else if (subtag_a[1] == "c" || subtag_a[1] == "custom") {
							st_parser_type = TextServer::STRUCTURED_TEXT_CUSTOM;
						}
					}
				}
			}
			push_paragraph(alignment, dir, lang, st_parser_type, jst_flags, tab_stops);
			pos = brk_end + 1;
			tag_stack.push_front("p");
		} else if (tag == "url") {
			int end = p_bbcode.find("[", brk_end);
			if (end == -1) {
				end = p_bbcode.length();
			}
			String url = p_bbcode.substr(brk_end + 1, end - brk_end - 1).unquote();
			push_meta(url);
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag.begins_with("url=")) {