### 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 (!(p_search_previous && char_idx < 0) &&
				_search_line(selection.from_frame, selection.from_line, p_string, char_idx, p_search_previous)) {
			scroll_to_selection();
			queue_redraw();
			return true;
		}
		char_idx = p_search_previous ? -1 : 0;
		
		if (selection.from_frame->parent != nullptr && selection.from_frame->parent->type == ITEM_TABLE) {
			
			ItemTable *parent_table = static_cast<ItemTable *>(selection.from_frame->parent);
			List<Item *>::Element *parent_element = p_search_previous ? parent_table->subitems.back() : parent_table->subitems.front();
			while (parent_element->get() != selection.from_frame) {
				parent_element = p_search_previous ? parent_element->prev() : parent_element->next();
				ERR_FAIL_COND_V(parent_element == nullptr, false);
			}
			
			if (!(p_search_previous && parent_element == parent_table->subitems.front()) &&
					parent_element != parent_table->subitems.back()) {
				parent_element = p_search_previous ? parent_element->prev() : parent_element->next(); 
				ERR_FAIL_COND_V(parent_element == nullptr, false);
				
				if (_search_table(parent_table, parent_element, p_string, p_search_previous)) {
					scroll_to_selection();
					queue_redraw();
					return true;
				}
			}
		}
		ending_line = selection.from_frame->line + selection.from_line;