Prevent schema corruption

The problem is quite simple.

If for some reason, a developper forget to add a "return $schema" or return a null value at the end of an hook_schema implementation, the cache 'schema' entry will be left empty at the next rebuild (update.php or when you clean the cache table). As this cache entry is used for every modules, this means that there is no schema available at all anymore (node, etc.) and result is an unusable site.

Drupal core should add a test for this :

--- drupal-6.4/includes/common.inc	2008-08-13 19:59:12.000000000 -0400
+++ drupal/includes/common.inc	2008-09-08 05:54:04.000000000 -0400
@@ -3058,8 +3058,10 @@
       // Invoke hook_schema for all modules.
       foreach (module_implements('schema') as $module) {
         $current = module_invoke($module, 'schema');
-        _drupal_initialize_schema($module, $current);
-        $schema = array_merge($schema, $current);
+        if (!empty($current)) {
+        	_drupal_initialize_schema($module, $current);
+        	$schema = array_merge($schema, $current);
+        }
       }