Unit test a method that returns a boolean

I have a method like this:

public bool CheckForRuleName(string ruleName, string studentId)
{
    var rules =
          _noSqlProvider.GetDocumentsByQuery<StudentRule>(string.Format(GET_Student_BASED_ON_RULENAME, ruleName, studentId));

    if (rules.Count() != 0)
    {
        return true;
    }
    return false;
}

Wrote a Unit test like so:

[TestMethod]
public void when_calling_CheckForRuleName_Should_Return_FALSE_WHEN_RULE_DOES_NOT_EXIST()
{
    var noSqlProv = new Mock<INoSqlProvider<Document, DocumentCollection>>();

    noSqlProv.Setup(x => x.GetDocumentsByQuery<StudentRule>(It.IsAny<string>()));

    var rulesRepository = new RulesRepository(noSqlProv.Object);
    bool rules = rulesRepository.CheckForRuleName("test123","testrule");

    Assert.AreEqual(false, rules);
}

The test is failing with the following exception:

ArgumentNullException: Value cannot be null.
Parameter name: source

Now the method GetDocumentsByQuery expects an SQL string like so:

"select j from json j where j.name='{0}' and j.studentid='{1}'"

It queries the DocumentDb, just to be complete on the question. Since it is not a good practice to a query the actual Db I used:

It.IsAny<string>()

I am new to unit testing so can someone please provide guidance on how to unit test this type of code.

Thanks in advance.

Regards.