fix: prevent TypeError in html_entity_decode with null values

- Add null coalescing operators (??) to handle null settings values
- Fixes TypeError: html_entity_decode(): Argument #1 ($string) must be of type string, null given
- Applied to notes_label and notes_placeholder settings across multiple methods
- Ensures PHP 7.4-8.4 compatibility maintained

Author: jakubmikita

readme.txt

@@ -70,6 +70,9 @@ Yes! We're offering a [custom plugin development](https://bracketspace.com/custo
 
 == Changelog ==
 
+= [Next] =
+* [Fixed] Added null coalescing operators to prevent PHP warnings when settings array keys don't exist.
+
 = 1.8.0 =
 * [Changed] Removed local translation files in favor of GlotPress repository.
 * [Changed] Coding standards to PSR-12.