1. Defined types
Typ A
property name
property reference 'ref'

Type B
property name
property reference 'ref'

2. Value in repository

type a, name 'something' , ref =empty (null)

3. Query:

SELECT a.* FROM [a] LEFT OUTER JOIN [b] ON a.[ref] = b.[jcr:uuid]

4. Result of query = empty, should be contained element

In source code:
NestedLoopJoinComponent class in case LEFT_OUTER/RIGH_OUTER:

Object leftValue = leftSelector.evaluate(leftTuple);
     if (leftValue == null) {
            continue;
     }

leftValue is null - because my ref is empty. continue is executed so nothing added to results.