Remove empty element after trimming them

This makes sure we do not accidently keeps empty element into the
recipients array.
Make sure numeric keys works

Right now, if you pass a non-keyed array as a recipients value, it does
not works. The empty calls returns true on the 0 key (index) and false
otherwise. This mixes up sendmail and make the delivery
non-deterministic.

I've replaced the empty calls with intval: this way, the can properly
distiguish key based and non-key based array.