### 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)

ERR_FAIL_COND_V(p_frame == nullptr, p_h);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), p_h);
	Line &l = p_frame->lines[p_line];
	MutexLock lock(l.text_buf->get_mutex());
	BitField<TextServer::LineBreakFlag> autowrap_flags = TextServer::BREAK_MANDATORY;
	switch (autowrap_mode) {
		case TextServer::AUTOWRAP_WORD_SMART:
			autowrap_flags = TextServer::BREAK_WORD_BOUND | TextServer::BREAK_ADAPTIVE | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_WORD:
			autowrap_flags = TextServer::BREAK_WORD_BOUND | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_ARBITRARY:
			autowrap_flags = TextServer::BREAK_GRAPHEME_BOUND | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_OFF:
			break;
	}
	autowrap_flags = autowrap_flags | TextServer::BREAK_TRIM_EDGE_SPACES;
	
	l.text_buf->clear();
	l.text_buf->set_break_flags(autowrap_flags);
	l.text_buf->set_justification_flags(_find_jst_flags(l.from));
	l.char_offset = *r_char_offset;
	l.char_count = 0;
	
	l.offset.x = _find_margin(l.from, p_base_font, p_base_font_size);
	l.text_buf->set_width(p_width - l.offset.x);
	l.text_buf->set_alignment(_find_alignment(l.from));