sql: remove some pointless allocations
sql: remove some pointless allocations

This commit removes the preallocation of empty left and right rows
during a join unless they're strictly necessary.

petermattis/cockroach,petermattis/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,petermattis/cockroach,petermattis/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,glycerine/cockroach,glycerine/cockroach,paperstreet/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,petermattis/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,maxlang/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach