### 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)

jst_flags.set_flag(TextServer::JUSTIFICATION_DO_NOT_SKIP_SINGLE_LINE);
							}
						}
					} else if (subtag_a[0] == "tab_stops") {
						Vector<String> splitters;
						splitters.push_back(",");
						splitters.push_back(";");
						tab_stops = subtag_a[1].split_floats_mk(splitters);
					} else if (subtag_a[0] == "align") {
						if (subtag_a[1] == "l" || subtag_a[1] == "left") {
							alignment = HORIZONTAL_ALIGNMENT_LEFT;
						} else if (subtag_a[1] == "c" || subtag_a[1] == "center") {
							alignment = HORIZONTAL_ALIGNMENT_CENTER;
						} else if (subtag_a[1] == "r" || subtag_a[1] == "right") {
							alignment = HORIZONTAL_ALIGNMENT_RIGHT;
						} else if (subtag_a[1] == "f" || subtag_a[1] == "fill") {
							alignment = HORIZONTAL_ALIGNMENT_FILL;
						}
					} else if (subtag_a[0] == "dir" || subtag_a[0] == "direction") {
						if (subtag_a[1] == "a" || subtag_a[1] == "auto") {
							dir = Control::TEXT_DIRECTION_AUTO;
						} else if (subtag_a[1] == "l" || subtag_a[1] == "ltr") {
							dir = Control::TEXT_DIRECTION_LTR;
						} else if (subtag_a[1] == "r" || subtag_a[1] == "rtl") {
							dir = Control::TEXT_DIRECTION_RTL;
						}
					} else if (subtag_a[0] == "lang" || subtag_a[0] == "language") {
						lang = subtag_a[1];
					} else if (subtag_a[0] == "st" || subtag_a[0] == "bidi_override") {
						if (subtag_a[1] == "d" || subtag_a[1] == "default") {