Fix deprecated usage of passing null to explode() (#37392)

As of PHP 8.1, explode() does not permit null as its second argument.
This results in warnings being spat out on every page because of a usage
of this in wp-includes/block-supports/layout.php.

lib/block-supports/layout.php

@@ -39,8 +39,8 @@ function gutenberg_get_layout_style( $selector, $layout, $has_block_gap_support
 
 	$style = '';
 	if ( 'default' === $layout_type ) {
-		$content_size = isset( $layout['contentSize'] ) ? $layout['contentSize'] : null;
-		$wide_size    = isset( $layout['wideSize'] ) ? $layout['wideSize'] : null;
+		$content_size = isset( $layout['contentSize'] ) ? $layout['contentSize'] : '';
+		$wide_size    = isset( $layout['wideSize'] ) ? $layout['wideSize'] : '';
 
 		$all_max_width_value  = $content_size ? $content_size : $wide_size;
 		$wide_max_width_value = $wide_size ? $wide_size : $content_size;