[JENA-79] Wrong query result when FILTERing an unbound variable - ASF JIRA

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Consider the graph:

@prefix ex: <http://example.com/ns#>.

ex:subject a ex:Class.

and the query

PREFIX ex: <http://example.com/ns#>
SELECT ?x WHERE {
  ?s a ?c
  OPTIONAL { ?s ex:property ?x }
  FILTER (?x = ex:v)
}

Executing the query on the graph should return no solutions (?x is unbound, so the filter evaluates to 'error'). But I get the result:

-----------------------------
| x                         |
=============================
| <http://example.com/ns#v> |
-----------------------------

When changing the query to filter on a literal (?x = "string") or (?x = 1), the query returns no results.