### 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);
						}
					}
				}
			}
			fc->set_base_font(font);
			if (def_font != CUSTOM_FONT) {
				_push_def_font_var(def_font, fc, fnt_size);
			} else {
				push_font(fc, fnt_size);
			}
			pos = brk_end + 1;
			tag_stack.push_front("font");
		} else if (tag.begins_with("outline_size=")) {
			int fnt_size = tag.substr(13, tag.length()).to_int();
			if (fnt_size > 0) {
				push_outline_size(fnt_size);
			}
			pos = brk_end + 1;
			tag_stack.push_front("outline_size");
		} else if (bbcode_name == "fade") {
			int start_index = 0;
			OptionMap::Iterator start_option = bbcode_options.find("start");
			if (start_option) {
				start_index = start_option->value.to_int();
			}
			int length = 10;
			OptionMap::Iterator length_option = bbcode_options.find("length");