### 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)

selection.from_item = nullptr;
								selection.from_char = 0;
								selection.to_frame = nullptr;
								selection.to_line = 0;
								selection.to_item = nullptr;
								selection.to_char = 0;
								deselect();
							}
						}
					}
				}
			} else if (b->is_pressed() && b->is_double_click() && selection.enabled) {
				
				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_frame) {
					const Line &l = c_frame->lines[c_line];
					MutexLock lock(l.text_buf->get_mutex());
					PackedInt32Array words = TS->shaped_text_get_word_breaks(l.text_buf->get_rid());
					for (int i = 0; i < words.size(); i = i + 2) {
						if (c_index >= words[i] && c_index < words[i + 1]) {
							selection.from_frame = c_frame;
							selection.from_line = c_line;
							selection.from_item = c_item;
							selection.from_char = words[i];