Update tests and perform check for background size before use.

lib/class-wp-theme-json-gutenberg.php

@@ -2400,8 +2400,9 @@ protected static function compute_style_properties( $styles, $settings = array()
 					$value = 'cover';
 				}
 				// If the background size is set to `contain` and no position is set, set the position to `center`.
-				if ( 'background-position' === $css_property && 'contain' === $styles['background']['backgroundSize'] ) {
-					$value = '50% 50%';
+				if ( 'background-position' === $css_property ) {
+					$background_size = $styles['background']['backgroundSize'] ?? null;
+					$value           = 'contain' === $background_size ? '50% 50%' : null;
 				}
 			}
 

phpunit/class-wp-theme-json-test.php

@@ -4898,6 +4898,8 @@ public function test_get_block_background_image_styles() {
 								'backgroundImage' => array(
 									'url' => 'http://example.org/verse.png',
 									'id'  => 123,
+									// Merged theme.json and global styles will retain the "ref" value.
+									'ref' => 'styles.background.backgroundImage',
 								),
 							),
 						),