### 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)

amplitude = amplitude_option->value.to_float();
			}
			float period = 5.0f;
			OptionMap::Iterator period_option = bbcode_options.find("freq");
			if (period_option) {
				period = period_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_wave(period, amplitude, connected);
			pos = brk_end + 1;
			tag_stack.push_front("wave");
			set_process_internal(true);
		} else if (bbcode_name == "tornado") {
			float radius = 10.0f;
			OptionMap::Iterator radius_option = bbcode_options.find("radius");
			if (radius_option) {
				radius = radius_option->value.to_float();
			}
			float frequency = 1.0f;
			OptionMap::Iterator frequency_option = bbcode_options.find("freq");
			if (frequency_option) {
				frequency = frequency_option->value.to_float();
			}
			bool connected = true;
			OptionMap::Iterator connected_option = bbcode_options.find("connected");
			if (connected_option) {