fix(stats): skip registry entries without keys

src/main/java/games/dmg/spigotevents/stats/BukkitRegistryKeys.java

@@ -7,7 +7,11 @@ final class BukkitRegistryKeys {
     private BukkitRegistryKeys() {}
 
     static NamespacedKey get(Keyed value) {
-        return value.getKey();
+        try {
+            return value.getKey();
+        } catch (IllegalArgumentException ignored) {
+            return null;
+        }
     }
 
     static String format(Keyed value, String fallback) {

src/test/java/games/dmg/spigotevents/stats/BukkitRegistryKeysTest.java

@@ -21,4 +21,13 @@ class BukkitRegistryKeysTest {
 
         assertNull(BukkitRegistryKeys.get(keyed));
     }
+
+    @Test
+    void reportsAKeyThatBukkitRejectsAsUnavailable() {
+        Keyed keyed = () -> {
+            throw new IllegalArgumentException("Registry entry does not have a key");
+        };
+
+        assertNull(BukkitRegistryKeys.get(keyed));
+    }
 }