### 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)

switch (p_option) {
		case MENU_COPY: {
			selection_copy();
		} break;
		case MENU_SELECT_ALL: {
			select_all();
		} break;
	}
}
void RichTextLabel::_draw_fbg_boxes(RID p_ci, RID p_rid, Vector2 line_off, Item *it_from, Item *it_to, int start, int end, int fbg_flag) {