### 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)

if (length_option) {
				length = length_option->value.to_int();
			}
			push_fade(start_index, length);
			pos = brk_end + 1;
			tag_stack.push_front("fade");
		} else if (bbcode_name == "shake") {
			int strength = 5;
			OptionMap::Iterator strength_option = bbcode_options.find("level");
			if (strength_option) {
				strength = strength_option->value.to_int();
			}
			float rate = 20.0f;
			OptionMap::Iterator rate_option = bbcode_options.find("rate");
			if (rate_option) {
				rate = rate_option->value.to_float();
			}
			bool connected = true;
			OptionMap::Iterator connected_option = bbcode_options.find("connected");
			if (connected_option) {
				connected = connected_option->value.to_int();
			}
			push_shake(strength, rate, connected);
			pos = brk_end + 1;
			tag_stack.push_front("shake");
			set_process_internal(true);
		} else if (bbcode_name == "wave") {
			float amplitude = 20.0f;
			OptionMap::Iterator amplitude_option = bbcode_options.find("amp");
			if (amplitude_option) {