NULL values in hook_menu() cause all menu builds to fail afterwards

I noticed this, while setting a menu item's description to NULL, but it will probably be the same for other fields defined as "not null" in the schema. Steps to reproduce:

- Install Drupal (of course)
- Enable the attached "Test module" (containing a menu item with a NULL description)
- Read the recent log messages to see that the menu_router_build() failed with an exception:
PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'description' cannot be null

Bad enough that this prevents ANY changes in menu router items afterwards, but it also somehow fails to set an error message. The reloaded Modules page looks exactly like normal, with a "The configuration options have been saved." status message.

Apparently, NULL descriptions are not very common, but when dynamically creating menu items, it can happen. And since the documentation of hook_menu() doesn't mention anything like this, I also didn't check for the case.

The problem is, _menu_router_build() tries to ensure non-NULL descriptions with the following code:

$item += array(
  …
  'description' => '',
  …
);

Descriptions that were explicitly set to NULL won't be affected by this (as will be the case for other fields).
Either we somehow fix this, or clearly state in the documentation that NULL values are not permitted. Otherwise, a single module innocently setting a NULL value could break a whole Drupal site, with the cause very hard to find.

The attached patch is a first go at fixing this, with a simple array_filter(). But more qualified solutions/patches are welcome. ^^"