[speculative] `NullStat`

[adienes]

I have some use cases where I might pass in any arbitrary OnlineStat type; occasionally I want this to be a no-op but not change the signature of the function

any receptiveness to adding a NullStat where fit!(::NullStat, x) simply does nothing?

just an idea

[joshday]

Seems a bit niche to add to the package, but it's pretty easy to do yourself:

import OnlineStatsBase: OnlineStat, _fit!, nobs, value
struct NullStat <: OnlineStat{Any} end 
value(::NullStat) = nothing 
nobs(::NullStat) = 0
_fit!(::NullStat, x) = nothing

julia> using OnlineStats

julia> fit!(NullStat(), randn(100))
NullStat: n=0 | value=nothing