OPTIONAL patterns breaks CONSTRUCT commands

[alien-mcl]

We've noticed an abnormal behawior with CONSTRUCT commands containing OPTIONAL patterns in the WHERE block. When store is fed with i.e. a resource which has no matching triples to meet the requirements of the OPTIONAL pattern, and variables from that pattern are used in the CONSTRUCT block, the result of such a command is empty. Example SPARQL:

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX lh: <http://localhost/resources/>
PREFIX voc: <http://localhost/vocabulary#>
CONSTRUCT {
    lh:test a voc:SetEnumeration .
    lh:test a voc:Command .
    lh:test voc:enumeration ?enumeration .
    ?enumeration rdf:predicate ?enumerationKey . 
    ?enumeration rdf:object ?enumerationValue . 
    lh:test voc:createdOn ?performedOn .
    lh:test voc:createdBy ?performedBy .
}
WHERE {
    lh:res a voc:SimpleDatatype .
    lh:res a voc:Datatype .
    OPTIONAL {
        lh:res voc:enumeration ?enumeration . 
        ?enumeration rdf:predicate ?enumerationKey . 
        ?enumeration rdf:object ?enumerationValue .
    }
    OPTIONAL { lh:res voc:createdOn ?performedOn . }
    OPTIONAL { lh:res voc:createdBy ?performedBy . }
}

JSON-LD resource feeding the store:

{
    "@id":"http://localhost/resources/res",
    "@type":["http://localhost/vocabulary#Datatype", "http://localhost/vocabulary#SimpleDatatype"],
    "http://localhost/vocabulary#type": [{
        "@type":"http://www.w3.org/2001/XMLSchema#anyUri",
        "@value":"http://www.w3.org/2001/XMLSchema#unsignedInt" }],
    "http://purl.org/dc/terms/identifier": [{
        "@type":"http://www.w3.org/2001/XMLSchema#NCName",
        "@value":"test" }],
    "http://www.w3.org/2000/01/rdf-schema#label": [{
        "@type":"http://www.w3.org/2001/XMLSchema#string",
        "@value":"Test" }],
    "http://localhost/vocabulary#enumeration": [{
        "http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate": [{"@value":"Test"}],
        "http://www.w3.org/1999/02/22-rdf-syntax-ns#object": [{"@value":"1"}]}]
}

Result of the store.execute is a success with an empty array of triples, while I expected something like this:

{
    "@id":"http://localhost/resources/test",
    "@type":["http://localhost/vocabulary#SetEnumeration", "http://localhost/vocabulary#Command"],
    "http://localhost/vocabulary#enumeration": [{
        "http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate": [{"@value":"Test"}],
        "http://www.w3.org/1999/02/22-rdf-syntax-ns#object": [{"@value":"1"}]}]
}

which I confirmed with other implementations of the SPARQL engines (dotNetRDF).