### 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)

accept_event();
			}
		}
	}
	Ref<InputEventMouseMotion> m = p_event;
	if (m.is_valid()) {
		ItemFrame *c_frame = nullptr;
		int c_line = 0;
		Item *c_item = nullptr;
		int c_index = 0;
		bool outside;
		_find_click(main, m->get_position(), &c_frame, &c_line, &c_item, &c_index, &outside, false);
		if (selection.click_item && c_item) {
			selection.from_frame = selection.click_frame;
			selection.from_line = selection.click_line;
			selection.from_item = selection.click_item;
			selection.from_char = selection.click_char;
			selection.to_frame = c_frame;
			selection.to_line = c_line;
			selection.to_item = c_item;
			selection.to_char = c_index;
			bool swap = false;
			if (selection.click_frame && c_frame) {
				const Line &l1 = c_frame->lines[c_line];
				const Line &l2 = selection.click_frame->lines[selection.click_line];
				if (l1.char_offset + c_index < l2.char_offset + selection.click_char) {
					swap = true;
				} else if (l1.char_offset + c_index == l2.char_offset + selection.click_char) {
					deselect();
					return;