### 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)

ERR_FAIL_COND(p_event.is_null());
	Ref<InputEventMouseButton> b = p_event;
	if (b.is_valid()) {
		if (b->get_button_index() == MouseButton::LEFT) {
			if (b->is_pressed() && !b->is_double_click()) {
				scroll_updated = false;
				ItemFrame *c_frame = nullptr;
				int c_line = 0;
				Item *c_item = nullptr;
				int c_index = 0;
				bool outside;
				selection.drag_attempt = false;
				_find_click(main, b->get_position(), &c_frame, &c_line, &c_item, &c_index, &outside, false);
				if (c_item != nullptr) {
					if (selection.enabled) {
						selection.click_frame = c_frame;
						selection.click_item = c_item;
						selection.click_line = c_line;
						selection.click_char = c_index;
						
						if (selection.active) {
							if (_is_click_inside_selection()) {
								selection.drag_attempt = true;
								selection.click_item = nullptr;
							} else {
								selection.from_frame = nullptr;
								selection.from_line = 0;