public static Object indexable(final Object obj) {
        return null == obj ? IndexedNull.instance() : obj;
    }
Provides a way for an index to have a {@code null} value as {@code ConcurrentHashMap} will not allow a
{@code null} key.
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/structure/AbstractTinkerIndex.java