Blocks: Don't return null for string value (#1766)

Author: obenland

.github/changelog/1766-from-description

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Avoid type mismatch when updating `activitypub_content_warning` meta values.

includes/class-blocks.php

@@ -43,13 +43,7 @@ public static function register_postmeta() {
 					'show_in_rest'      => true,
 					'single'            => true,
 					'type'              => 'string',
-					'sanitize_callback' => function ( $warning ) {
-						if ( $warning ) {
-							return \sanitize_text_field( $warning );
-						}
-
-						return null;
-					},
+					'sanitize_callback' => 'sanitize_text_field',
 				)
 			);