Fixes: Anvil Rename Bug + Arcane Archer Variant

1) Anvil Rename Bug (vanilla renaming broken)

Problem

Players couldn’t rename items in the anvil.
The result item would appear for a second and then disappear.

Cause

The anvil listener was clearing the result whenever either input slot was empty:

• Vanilla rename uses only the left slot (right slot is empty).
• The plugin logic treated right == null as invalid and forced event.setResult(null).

Fix

Allow vanilla anvil operations when the right slot is empty.

Before (buggy logic)

if (left == null || right == null) {
    event.setResult(null);
    return;
}