### 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)

font_outline_color.a = faded_visibility;
				font_shadow_color.a = faded_visibility;
			}
			bool txt_visible = (font_outline_color.a != 0) || (font_shadow_color.a != 0);
			for (int j = 0; j < fx_stack.size(); j++) {
				ItemFX *item_fx = fx_stack[j];
				bool cn = cprev_cluster || (cprev_conn && item_fx->connected);
				if (item_fx->type == ITEM_CUSTOMFX && custom_fx_ok) {
					ItemCustomFX *item_custom = static_cast<ItemCustomFX *>(item_fx);
					Ref<CharFXTransform> charfx = item_custom->char_fx_transform;
					Ref<RichTextEffect> custom_effect = item_custom->custom_effect;
					if (!custom_effect.is_null()) {
						charfx->elapsed_time = item_custom->elapsed_time;
						charfx->range = Vector2i(l.char_offset + glyphs[i].start, l.char_offset + glyphs[i].end);
						charfx->relative_index = l.char_offset + glyphs[i].start - item_fx->char_ofs;
						charfx->visibility = txt_visible;
						charfx->outline = true;
						charfx->font = frid;
						charfx->glyph_index = gl;
						charfx->glyph_flags = gl_fl;
						charfx->glyph_count = gl_cn;
						charfx->offset = fx_offset;
						charfx->color = font_color;
						bool effect_status = custom_effect->_process_effect_impl(charfx);
						custom_fx_ok = effect_status;
						fx_offset += charfx->offset;
						font_color = charfx->color;
						frid = charfx->font;
						gl = charfx->glyph_index;
						txt_visible &= charfx->visibility;