### 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 ((int)p_frame->lines.size() - 1 <= p_frame->first_invalid_line) {
		p_frame->first_invalid_line = (int)p_frame->lines.size() - 1;
	}
}
void RichTextLabel::add_text(const String &p_text) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	if (current->type == ITEM_TABLE) {
		return; 
	}
	int pos = 0;
	while (pos < p_text.length()) {
		int end = p_text.find("\n", pos);
		String line;
		bool eol = false;
		if (end == -1) {
			end = p_text.length();
		} else {
			eol = true;
		}
		if (pos == 0 && end == p_text.length()) {
			line = p_text;
		} else {