### 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)

color = Color::from_string(color_option->value, color);
			}
			float frequency = 1.0;
			OptionMap::Iterator freq_option = bbcode_options.find("freq");
			if (freq_option) {
				frequency = freq_option->value.to_float();
			}
			float ease = -2.0;
			OptionMap::Iterator ease_option = bbcode_options.find("ease");
			if (ease_option) {
				ease = ease_option->value.to_float();
			}
			push_pulse(color, frequency, ease);
			pos = brk_end + 1;
			tag_stack.push_front("pulse");
			set_process_internal(true);
		} else if (tag.begins_with("bgcolor=")) {
			String color_str = tag.substr(8, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_bgcolor(color);
			pos = brk_end + 1;
			tag_stack.push_front("bgcolor");
		} else if (tag.begins_with("fgcolor=")) {
			String color_str = tag.substr(8, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_fgcolor(color);
			pos = brk_end + 1;
			tag_stack.push_front("fgcolor");
		} else {
			Vector<String> &expr = split_tag_block;