### 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)

set_text(text);
			}
			main->first_invalid_line.store(0); 
			queue_redraw();
		} break;
		case NOTIFICATION_PREDELETE:
		case NOTIFICATION_EXIT_TREE: {
			_stop_thread();
		} break;
		case NOTIFICATION_LAYOUT_DIRECTION_CHANGED:
		case NOTIFICATION_TRANSLATION_CHANGED: {
			_apply_translation();
			queue_redraw();
		} break;
		case NOTIFICATION_INTERNAL_PHYSICS_PROCESS: {
			if (is_visible_in_tree()) {
				queue_redraw();
			}
		} break;
		case NOTIFICATION_DRAW: {
			RID ci = get_canvas_item();
			Size2 size = get_size();
			draw_style_box(theme_cache.normal_style, Rect2(Point2(), size));
			if (has_focus()) {
				RenderingServer::get_singleton()->canvas_item_add_clip_ignore(ci, true);
				draw_style_box(theme_cache.focus_style, Rect2(Point2(), size));
				RenderingServer::get_singleton()->canvas_item_add_clip_ignore(ci, false);
			}
			
			if (_validate_line_caches()) {