fix trivial @accessor functions

[aplavin]

@accessor f(x) = x triggered some edgecase in parsing, fixed now.

Can be reproduced with regular @optic macros without @accessor, but probably noone write optics like this:

# worked before as well:
julia> @optic begin
       _ + 1
       end
(::Base.Fix2{typeof(+), Int64}) (generic function with 1 method)

# threw an error before:
julia> @optic begin
       _
       end
identity (generic function with 1 method)