### 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)

if (subtag[2] == "top" || subtag[2] == "t") {
					alignment |= INLINE_ALIGNMENT_TO_TOP;
				} else if (subtag[2] == "center" || subtag[2] == "c") {
					alignment |= INLINE_ALIGNMENT_TO_CENTER;
				} else if (subtag[2] == "baseline" || subtag[2] == "l") {
					alignment |= INLINE_ALIGNMENT_TO_BASELINE;
				} else if (subtag[2] == "bottom" || subtag[2] == "b") {
					alignment |= INLINE_ALIGNMENT_TO_BOTTOM;
				}
			} else if (subtag.size() > 1) {
				if (subtag[1] == "top" || subtag[1] == "t") {
					alignment = INLINE_ALIGNMENT_TOP;
				} else if (subtag[1] == "center" || subtag[1] == "c") {
					alignment = INLINE_ALIGNMENT_CENTER;
				} else if (subtag[1] == "bottom" || subtag[1] == "b") {
					alignment = INLINE_ALIGNMENT_BOTTOM;
				}
			}
			int row = -1;
			if (subtag.size() > 3) {
				row = subtag[3].to_int();
			}
			push_table(columns, (InlineAlignment)alignment, row);
			pos = brk_end + 1;
			tag_stack.push_front("table");
		} else if (tag == "cell") {
			push_cell();
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag.begins_with("cell=")) {