### 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)

pos = brk_end + 1;
			tag_stack.push_front("font_size");
		} else if (tag.begins_with("opentype_features=") || tag.begins_with("otf=")) {
			int value_pos = tag.find("=");
			String fnt_ftr = tag.substr(value_pos + 1);
			Vector<String> subtag = fnt_ftr.split(",");
			_normalize_subtags(subtag);
			if (subtag.size() > 0) {
				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;
					}
				}
				Dictionary features;
				for (int i = 0; i < subtag.size(); i++) {
					Vector<String> subtag_a = subtag[i].split("=");
					_normalize_subtags(subtag_a);
					if (subtag_a.size() == 2) {
						features[TS->name_to_tag(subtag_a[0])] = subtag_a[1].to_int();
					} else if (subtag_a.size() == 1) {
						features[TS->name_to_tag(subtag_a[0])] = 1;
					}
				}
				Ref<FontVariation> fc;
				fc.instantiate();
				fc->set_base_font(font);