### 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)

a.append(Color::html(values[j]));
			} else if (!nodepath.search(values[j]).is_null()) {
				if (values[j].begins_with("$")) {
					String v = values[j].substr(1, values[j].length());
					a.append(NodePath(v));
				}
			} else if (!boolean.search(values[j]).is_null()) {
				if (values[j] == "true") {
					a.append(true);
				} else if (values[j] == "false") {
					a.append(false);
				}
			} else if (!decimal.search(values[j]).is_null()) {
				a.append(values[j].to_float());
			} else if (!numerical.search(values[j]).is_null()) {
				a.append(values[j].to_int());
			} else {
				a.append(values[j]);
			}
		}
#endif
		if (values.size() > 1) {
			d[key] = a;
		} else if (values.size() == 1) {
			d[key] = a[0];
		}
	}
	return d;
}
RichTextLabel::RichTextLabel(const String &p_text) {