### 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)

cprev_cluster = true;
			}
			if (gl_fl & TextServer::GRAPHEME_IS_RTL) { 
				if (i > 0 && (glyphs[i - 1].flags & TextServer::GRAPHEME_IS_CONNECTED)) {
					cprev_conn = true;
				}
			} else {
				if (glyphs[i].flags & TextServer::GRAPHEME_IS_CONNECTED) {
					cprev_conn = true;
				}
			}
			
			if (fade) {
				float faded_visibility = 1.0f;
				if (glyphs[i].start >= fade->starting_index) {
					faded_visibility -= (float)(glyphs[i].start - fade->starting_index) / (float)fade->length;
					faded_visibility = faded_visibility < 0.0f ? 0.0f : faded_visibility;
				}
				font_color.a = faded_visibility;
			}
			bool txt_visible = (font_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;