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.

git-svn-id: https://develop.svn.wordpress.org/trunk@35554 602fd350-edb4-49c9-b593-d223f7449a82

src/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.