### 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(!selection.enabled, false);
	if (p_string.size() == 0) {
		selection.active = false;
		return false;
	}
	int char_idx = p_search_previous ? -1 : 0;
	int current_line = 0;
	int to_line = main->first_invalid_line.load();
	int ending_line = to_line - 1;
	if (p_from_selection && selection.active) {
		
		char_idx = p_search_previous ? selection.from_char - 1 : selection.to_char;