pics:
constant combinator - no values set at all, just something to connect the wire to

arithmetic combiner - non-existant '0' incremented and output

(skipping the light, it's just set to '0'>0
as you can see, with nothing wired into input on the combiner, it outputs nothing and the light is off...

wire it to the constant, which is still not outputting a signal on '0', but suddenly, it accepts the implied '0' as 0

I used a constant combinator with no values set, but wiring to anything seems to work - I tried a power pole, a chest, a smart inserter, all had the same behavior.
Obviously this is a minor thing, I figure it's either a 10 minute fix or going to be bumped to "not fixing" for some non-obvious reason, and I won't lose sleep either way, just thought I'd report it; this sort of inconsistency can potentially lead to very confusing situations.
:edit: issue (if you wanna call it that) still present after 0.12.1 update