### 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)

fc.instantiate();
			int fnt_size = -1;
			for (int i = 1; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=", true, 1);
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "name" || subtag_a[0] == "n") {
						String fnt = subtag_a[1];
						Ref<Font> font_data = ResourceLoader::load(fnt, "Font");
						if (font_data.is_valid()) {
							font = font_data;
							def_font = CUSTOM_FONT;
						}
					} else if (subtag_a[0] == "size" || subtag_a[0] == "s") {
						fnt_size = subtag_a[1].to_int();
					} else if (subtag_a[0] == "glyph_spacing" || subtag_a[0] == "gl") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_GLYPH, spacing);
					} else if (subtag_a[0] == "space_spacing" || subtag_a[0] == "sp") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_SPACE, spacing);
					} else if (subtag_a[0] == "top_spacing" || subtag_a[0] == "top") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_TOP, spacing);
					} else if (subtag_a[0] == "bottom_spacing" || subtag_a[0] == "bt") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_BOTTOM, spacing);
					} else if (subtag_a[0] == "embolden" || subtag_a[0] == "emb") {
						float emb = subtag_a[1].to_float();
						fc->set_variation_embolden(emb);