### 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);
	if (current->type == ITEM_TABLE) {
		return;
	}
	ERR_FAIL_COND(p_image.is_null());
	ERR_FAIL_COND(p_image->get_width() == 0);
	ERR_FAIL_COND(p_image->get_height() == 0);
	ItemImage *item = memnew(ItemImage);
	if (p_region.has_area()) {
		Ref<AtlasTexture> atlas_tex = memnew(AtlasTexture);
		atlas_tex->set_atlas(p_image);
		atlas_tex->set_region(p_region);
		item->image = atlas_tex;
	} else {
		item->image = p_image;
	}
	item->color = p_color;
	item->inline_align = p_alignment;
	if (p_width > 0) {
		
		item->size.width = p_width;
		if (p_height > 0) {
			
			item->size.height = p_height;
		} else {