Specify the expected behaviour when calling distance with varying parameters

It is unclear what is expected when calling the distance function with unusual parameters...

1. When passing a nodeset it is possible for the expression to resolve 0 or 1 nodes. Prior art: Collect returns 0. Web Forms returns NaN.
2. When passing multiple params when one of them is empty, for example, `distance("1 1", null, "2 2")`. Prior art: Collect treats the null as "0 0" and adds "1 1" -> "0 0", and "0 0" -> "2 2". Web Forms ignores the null and returns "1 1" -> "2 2".

Start by reviewing the distance function and the cited Collect and Web Forms behavior. The work is done when the expected results for nodesets resolving to zero or one node, and for empty parameters among multiple arguments, are explicitly specified.

Written by the indexing model from the issue text.