### 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)

int ratio = tag.substr(5, tag.length()).to_int();
			if (ratio < 1) {
				ratio = 1;
			}
			set_table_column_expand(get_current_table_column(), true, ratio);
			push_cell();
			pos = brk_end + 1;
			tag_stack.push_front("cell");
		} else if (tag.begins_with("cell ")) {
			Vector<String> subtag = tag.substr(5, tag.length()).split(" ");
			_normalize_subtags(subtag);
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "expand") {
						int ratio = subtag_a[1].to_int();
						if (ratio < 1) {
							ratio = 1;
						}
						set_table_column_expand(get_current_table_column(), true, ratio);
					}
				}
			}
			push_cell();
			const Color fallback_color = Color(0, 0, 0, 0);
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {