### 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);
	ItemShake *item = memnew(ItemShake);
	item->strength = p_strength;
	item->rate = p_rate;
	item->connected = p_connected;
	_add_item(item, true);
}
void RichTextLabel::push_wave(float p_frequency = 1.0f, float p_amplitude = 10.0f, bool p_connected = true) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemWave *item = memnew(ItemWave);
	item->frequency = p_frequency;
	item->amplitude = p_amplitude;
	item->connected = p_connected;
	_add_item(item, true);
}
void RichTextLabel::push_tornado(float p_frequency = 1.0f, float p_radius = 10.0f, bool p_connected = true) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemTornado *item = memnew(ItemTornado);
	item->frequency = p_frequency;
	item->radius = p_radius;
	item->connected = p_connected;
	_add_item(item, true);
}
void RichTextLabel::push_rainbow(float p_saturation, float p_value, float p_frequency) {