### 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)

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;
				}
				String lang = _find_language(it);
				String tx = t->text;
				if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING && visible_characters >= 0 && remaining_characters >= 0) {
					tx = tx.substr(0, remaining_characters);
				}
				remaining_characters -= tx.length();
				l.text_buf->add_string(tx, font, font_size, lang, (uint64_t)it);
				txt += tx;
				l.char_count += tx.length();
			} break;
			case ITEM_IMAGE: {
				ItemImage *img = static_cast<ItemImage *>(it);
				l.text_buf->add_object((uint64_t)it, img->size, img->inline_align, 1);
				txt += String::chr(0xfffc);
				l.char_count++;
				remaining_characters--;
			} break;
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);
				int col_count = table->columns.size();
				int t_char_count = 0;
				
				for (int i = 0; i < col_count; i++) {
					table->columns[i].min_width = 0;
					table->columns[i].max_width = 0;
					table->columns[i].width = 0;