public static <T> Collection<T> leftJoin(Collection<T> coll) {
        if (coll == null || coll.isEmpty()) {
            @SuppressWarnings("unchecked") // List only contains null
            Collection<T> rv = (Collection<T>) nullList;
            return rv;
        } else {
            return coll;
        }
    }
{@code CollQueryFunctions} defines function implementation for use in ColQueryTemplates

@author tiwe
querydsl-collections/src/main/java/com/querydsl/collections/CollQueryFunctions.java