### 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)

return visible_ratio;
}
void RichTextLabel::set_effects(Array p_effects) {
	custom_effects = p_effects;
	if ((!text.is_empty()) && use_bbcode) {
		parse_bbcode(atr(text));
	}
}
Array RichTextLabel::get_effects() {
	return custom_effects;
}
void RichTextLabel::install_effect(const Variant effect) {
	Ref<RichTextEffect> rteffect;
	rteffect = effect;
	ERR_FAIL_COND_MSG(rteffect.is_null(), "Invalid RichTextEffect resource.");
	custom_effects.push_back(effect);
	if ((!text.is_empty()) && use_bbcode) {
		parse_bbcode(atr(text));
	}
}
int RichTextLabel::get_content_height() const {