Explicit boxing shows that NPE may occur so document it

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1477738 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/collections4/trie/CharacterKeyAnalyzer.java

@@ -65,9 +65,10 @@ public class CharacterKeyAnalyzer extends AbstractKeyAnalyzer<Character> {
 
     /**
      * {@inheritDoc}
+     * @throws NullPointerException if the key is null
      */
     public boolean isBitSet(final Character key, final int bitIndex, final int lengthInBits) {
-        return (key & mask(bitIndex)) != 0;
+        return (key.charValue() & mask(bitIndex)) != 0;
     }
 
     /**