### 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)

Dictionary d;
	for (int i = 0; i < p_expressions.size(); i++) {
		String expression = p_expressions[i];
		Array a;
		Vector<String> parts = expression.split("=", true);
		String key = parts[0];
		if (parts.size() != 2) {
			return d;
		}
		Vector<String> values = parts[1].split(",", false);
#ifdef MODULE_REGEX_ENABLED
		RegEx color = RegEx();
		color.compile("^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$");
		RegEx nodepath = RegEx();
		nodepath.compile("^\\$");
		RegEx boolean = RegEx();
		boolean.compile("^(true|false)$");
		RegEx decimal = RegEx();
		decimal.compile("^-?^.?\\d+(\\.\\d+?)?$");
		RegEx numerical = RegEx();
		numerical.compile("^\\d+$");
		for (int j = 0; j < values.size(); j++) {
			if (!color.search(values[j]).is_null()) {