### 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)

_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemRainbow *item = memnew(ItemRainbow);
	item->frequency = p_frequency;
	item->saturation = p_saturation;
	item->value = p_value;
	_add_item(item, true);
}
void RichTextLabel::push_pulse(const Color &p_color, float p_frequency, float p_ease) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ItemPulse *item = memnew(ItemPulse);
	item->color = p_color;
	item->frequency = p_frequency;
	item->ease = p_ease;
	_add_item(item, true);
}
void RichTextLabel::push_bgcolor(const Color &p_color) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemBGColor *item = memnew(ItemBGColor);
	item->color = p_color;
	_add_item(item, true);
}
void RichTextLabel::push_fgcolor(const Color &p_color) {