[C] Reduce XP for empty enchants. Fixes BUKKIT-5557

### The Issue:

Plugins that handle the ItemEnchantEvent by removing default enchantments will no longer have XP consumed. This is an API behavior change due to a semi-recent commit.

### Justifications for this PR:

Restoring the enchanting event behavior that existed prior to 1.7.2 R0-3

### Testing Results:

I tested this fix with various vanilla enchanting combinations, including different types of items, to make sure that vanilla enchanting behavior was preserved. I also tested with a plugin that handles ItemEnchantEvent by returning an empty enchant list, and XP was deducted as expected.

### Testing Material

I have made a plugin for testing this fix:

https://github.com/NathanWolf/Test-Bukkit-5557/
http://mine.elmakers.com/Test-Bukkit-5777-0.1-SNAPSHOT.jar

To test:

1. Install the test plugin
2. Get a diamond sword
3. Enchant the diamond sword

You will see the plugin intercept the enchant event and print a message. Without the fix, XP will not be deducted for this event, and it can be repeated over and over.

With the fix (or prior to 1.7.2-R0.3), clicking the button will deduct XP, and the button will grey out (unless you still have enough XP to enchant again).

Since commit ac1a2d0, clicking the button no longer deducts XP- however the plugin is not aware of this, and doesn't know it should be deducting the XP itself.

### JIRA Ticket:

BUKKIT-5557 - https://bukkit.atlassian.net/browse/BUKKIT-5557

### Related

#1366