### 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)

fc->set_opentype_features(features);
				if (def_font != CUSTOM_FONT) {
					_push_def_font_var(def_font, fc);
				} else {
					push_font(fc);
				}
			}
			pos = brk_end + 1;
			tag_stack.push_front(tag.substr(0, value_pos));
		} else if (tag.begins_with("font=")) {
			String fnt = tag.substr(5, tag.length()).unquote();
			Ref<Font> fc = ResourceLoader::load(fnt, "Font");
			if (fc.is_valid()) {
				push_font(fc);
			}
			pos = brk_end + 1;
			tag_stack.push_front("font");
		} else if (tag.begins_with("font ")) {
			Vector<String> subtag = tag.substr(2, tag.length()).split(" ");
			_normalize_subtags(subtag);
			Ref<Font> font = theme_cache.normal_font;
			DefaultFont def_font = NORMAL_FONT;
			ItemFont *font_it = _find_font(current);
			if (font_it) {
				if (font_it->font.is_valid()) {
					font = font_it->font;
					def_font = font_it->def_font;
				}
			}
			Ref<FontVariation> fc;