[v2] use of groupBy with vars returns null results

[aaj3f]

Description

Using groupBy with a ?binding that is also a ?binding in the vars map causes the query to return null results.

For example, the following query:

{
    "selectDistinct": [
        "?studentName",
        "?studentId"
    ],
    "where": [
      ["?profile", "profile/name", "?studentName"],
      ["?profile", "profile/student_id", "?studentId"]
    ],
    "vars": {
        "?studentName": ["Gates Lowery", "Kate Giovacchini"]
    },
    "opts": {
      "groupBy": "?studentName"
    }
}

// =>

null

However, this same query would work were you to remove either the vars map or the groupBy opt.