array:partition → fn:partition: empty results; examples

[ChristianGruen]

1. I think that an empty sequence should be returned if array:partition is invoked with an empty array. See e.g. fn:tokenize(''), which also returns an empty sequence and no zero-character string.

2. The last two example queries need to be revised, e.g. as follows:

array:partition(
  tokenize("In the beginning was the word"),
  function($previous, $current) { sum(($previous, $current) ! string-length()) gt 10 }
)

array:partition(
  (1, 2, 3, 6, 7, 9, 10),
  function($seq, $new) { not($new = $seq[last()] + 1) }
)

Maybe an affirming $add-when function is more intuitive than $break-when, in particular for function bodies with sequence operations and general comparisons.

[michaelhkay]

I believe that PR #454 has addressed the problems identified, and am therefore closing the issue.