### 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)

String url = tag.substr(4, tag.length()).unquote();
			push_meta(url);
			pos = brk_end + 1;
			tag_stack.push_front("url");
		} else if (tag.begins_with("hint=")) {
			String description = tag.substr(5, tag.length()).unquote();
			push_hint(description);
			pos = brk_end + 1;
			tag_stack.push_front("hint");
		} else if (tag.begins_with("dropcap")) {
			Vector<String> subtag = tag.substr(5, tag.length()).split(" ");
			_normalize_subtags(subtag);
			int fs = theme_cache.normal_font_size * 3;
			Ref<Font> f = theme_cache.normal_font;
			Color color = theme_cache.default_color;
			Color outline_color = theme_cache.font_outline_color;
			int outline_size = theme_cache.outline_size;
			Rect2 dropcap_margins;
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "font" || subtag_a[0] == "f") {
						String fnt = subtag_a[1];
						Ref<Font> font = ResourceLoader::load(fnt, "Font");
						if (font.is_valid()) {
							f = font;
						}
					} else if (subtag_a[0] == "font_size") {
						fs = subtag_a[1].to_int();