Better support export workflows which omit the Select page

Various code, my earlier patches and upstream Civi, were taking optional
fields in $exportParams for granted.  There was also an egregiously wrong
usage of array_search; it's likely this bug has been dormant since last
April.

Change-Id: I7452e8c647cc099be8643e5b315205ee26049ff1
Merge "Better support export workflows which omit the Select page"