### 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)

tag_stack.push_front("p");
		} else if (tag.begins_with("p ")) {
			Vector<String> subtag = tag.substr(2, tag.length()).split(" ");
			_normalize_subtags(subtag);
			HorizontalAlignment alignment = HORIZONTAL_ALIGNMENT_LEFT;
			Control::TextDirection dir = Control::TEXT_DIRECTION_INHERITED;
			String lang;
			PackedFloat32Array tab_stops;
			TextServer::StructuredTextParser st_parser_type = TextServer::STRUCTURED_TEXT_DEFAULT;
			BitField<TextServer::JustificationFlag> jst_flags = default_jst_flags;
			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] == "justification_flags" || subtag_a[0] == "jst") {
						Vector<String> subtag_b = subtag_a[1].split(",");
						for (const String &E : subtag_b) {
							if (E == "kashida" || E == "k") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_KASHIDA);
							} else if (E == "word" || E == "w") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_WORD_BOUND);
							} else if (E == "trim" || E == "tr") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_TRIM_EDGE_SPACES);
							} else if (E == "after_last_tab" || E == "lt") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_AFTER_LAST_TAB);
							} else if (E == "skip_last" || E == "sl") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_SKIP_LAST_LINE);
							} else if (E == "skip_last_with_chars" || E == "sv") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_SKIP_LAST_LINE_WITH_VISIBLE_CHARS);
							} else if (E == "do_not_skip_singe" || E == "ns") {