return null for first, last, nth on error

[sooda]

Instead of empty string, return null value that's considered undefined
(and can be piped to the default filter) if the array is empty for the
filters first, last and nth. For nth, also change the out-of-bounds
value from empty string to null.

Closes #534, and is a breaking change.

[Keats]

Thanks!

[sooda]

(Found some related uncommitted changes on documentation, so force-pushed a new revision not to forget them.)