### 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)

Item *it = p_frame;
	while (it) {
		ItemFX *ifx = nullptr;
		if (it->type == ITEM_CUSTOMFX || it->type == ITEM_SHAKE || it->type == ITEM_WAVE || it->type == ITEM_TORNADO || it->type == ITEM_RAINBOW || it->type == ITEM_PULSE) {
			ifx = static_cast<ItemFX *>(it);
		}
		if (!ifx) {
			it = _get_next_item(it, true);
			continue;
		}
		ifx->elapsed_time += p_delta_time;
		ItemShake *shake = nullptr;
		if (it->type == ITEM_SHAKE) {
			shake = static_cast<ItemShake *>(it);
		}
		if (shake) {
			bool cycle = (shake->elapsed_time > (1.0f / shake->rate));
			if (cycle) {
				shake->elapsed_time -= (1.0f / shake->rate);
				shake->reroll_random();
			}
		}
		it = _get_next_item(it, true);
	}
}
int RichTextLabel::_find_first_line(int p_from, int p_to, int p_vofs) const {