@Override
    public String getLocalKey() {
        if (prefixComponentIndex < prefix.size())
            return prefix.get(prefixComponentIndex);
        else
            return null;
    }
Returns the correct prefix component if we are still going down the prefix path,
or null to get all if we are inside the prefix
container-search/src/main/java/com/yahoo/search/query/profile/PrefixQueryProfileVisitor.java