Writer field: fix content with non-breaking space

Fixes #6285

config/fields/writer.php

@@ -63,7 +63,14 @@
 	'computed' => [
 		'value' => function () {
 			$value = trim($this->value ?? '');
-			return Sane::sanitize($value, 'html');
+			$value = Sane::sanitize($value, 'html');
+
+			// convert non-breaking spaces to HTML entity
+			// as that's how ProseMirror handles it internally;
+			// will allow comparing saved and current content
+			$value = str_replace(' ', ' ', $value);
+
+			return $value;
 		}
 	],
 	'validations' => [