### 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)

end = p_bbcode.length();
			}
			String image = p_bbcode.substr(brk_end + 1, end - brk_end - 1);
			Ref<Texture2D> texture = ResourceLoader::load(image, "Texture2D");
			if (texture.is_valid()) {
				Rect2 region;
				OptionMap::Iterator region_option = bbcode_options.find("region");
				if (region_option) {
					Vector<String> region_values = region_option->value.split(",", false);
					if (region_values.size() == 4) {
						region.position.x = region_values[0].to_float();
						region.position.y = region_values[1].to_float();
						region.size.x = region_values[2].to_float();
						region.size.y = region_values[3].to_float();
					}
				}
				Color color = Color(1.0, 1.0, 1.0);
				OptionMap::Iterator color_option = bbcode_options.find("color");
				if (color_option) {
					color = Color::from_string(color_option->value, color);
				}
				int width = 0;
				int height = 0;
				if (!bbcode_value.is_empty()) {
					int sep = bbcode_value.find("x");
					if (sep == -1) {
						width = bbcode_value.to_int();
					} else {
						width = bbcode_value.substr(0, sep).to_int();
						height = bbcode_value.substr(sep + 1).to_int();