### 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)

if (font_it) {
					if (font_it->font.is_valid()) {
						font = font_it->font;
					}
					if (font_it->font_size > 0) {
						font_size = font_it->font_size;
					}
				}
				ItemFontSize *font_size_it = _find_font_size(it);
				if (font_size_it && font_size_it->font_size > 0) {
					font_size = font_size_it->font_size;
				}
				l.text_buf->add_string("\n", font, font_size);
				txt += "\n";
				l.char_count++;
				remaining_characters--;
			} break;
			case ITEM_TEXT: {
				ItemText *t = static_cast<ItemText *>(it);
				Ref<Font> font = p_base_font;
				int font_size = p_base_font_size;
				ItemFont *font_it = _find_font(it);
				if (font_it) {
					if (font_it->font.is_valid()) {
						font = font_it->font;
					}
					if (font_it->font_size > 0) {
						font_size = font_it->font_size;
					}
				}