fix sparql count & sample for unbound

[joernhees]

fixes #563 and builds on top of #564, changing it to return None in case SAMPLE(?var) can't find a binding for ?var.

[joernhees]

@gromgull hehe, well just out of curiosity i asked jena:

$ touch empty.n3
$ sparql --data empty.n3 --query <(echo 'SELECT ?x (SAMPLE(?y) as ?ys) (SAMPLE(?z) as ?zs) WHERE {
  VALUES (?x ?y ?z) {
    (2 6 UNDEF)
    (2 UNDEF 10)
    (3 UNDEF 15)
    (3 9 UNDEF)
    (5 UNDEF 25)
  }
}
GROUP BY ?x
')
---------------
| x | ys | zs |
===============
| 5 |    | 25 |
| 3 |    |    |
| 2 |    |    |
---------------

[joernhees]

also see https://lists.w3.org/Archives/Public/public-sparql-dev/2016JanMar/0005.html

[joernhees]

sooo... according to @afs in https://lists.w3.org/Archives/Public/public-sparql-dev/2016JanMar/0006.html both our sample behaviors are correct, but the new one is a bit more useful. the old behavior wrt. count definitely was a bug.