Media: Avoid a PHP warning in `wp_generate_attachment_metadata()` if `$metadata` is not an array.

This was previously fixed in [25968], but accidentally reverted in [32545].

Props skithund.
Fixes #34599.
Built from https://develop.svn.wordpress.org/trunk@35554


git-svn-id: http://core.svn.wordpress.org/trunk@35518 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/image.php

@@ -194,7 +194,9 @@ function wp_generate_attachment_metadata( $attachment_id, $file ) {
 	}
 
 	// Remove the blob of binary data from the array.
-	unset( $metadata['image']['data'] );
+	if ( $metadata ) {
+		unset( $metadata['image']['data'] );
+	}
 
 	/**
 	 * Filter the generated attachment meta data.

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta3-35553';
+$wp_version = '4.4-beta3-35554';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.