sum-each, mul-each: handle no vars case.

* stdlib/arith-each.tl (sys:arith-each): If there are no vars,
then just reduce to the identity element value.
This is alreading happening fine for the each-prod family
of operators.

* tests/016/arith.tl: Test cases covering the no vars
and empty iteration identity element cases for sum-each and
mul-each, as well as the *-prod variants.

* txr.1: Document empty iteration and empty vars behavior
for arithmetic each operators as well as the each-prod
family.

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