Fix dict replcompletion error without key

[pfitzseb]

This fixes an error triggered by trying to autocomplete in a Dicts getindex call.

Before:

julia> foo = Dict(:bar => 3)
Dict{Symbol,Int64} with 1 entry:
  :bar => 3

julia> foo[┌ Error: Error in the keymap
│   exception =
│    MethodError: no method matching iterate(::Nothing)
│    Closest candidates are:
│      iterate(::Any) at essentials.jl:821
│      iterate(::Any, ::Any) at essentials.jl:816
│      iterate(::Core.SimpleVector) at essentials.jl:552
│      ...
│    Stacktrace:
│     [1] start(::Nothing) at ./essentials.jl:843
│     [2] iterate at ./essentials.jl:821 [inlined]
│     [3] first(::Nothing) at ./abstractarray.jl:287
│     [4] dict_identifier_key(::String, ::Symbol) ...

After:

julia> foo = Dict(:bar => 3)
Dict{Symbol,Int64} with 1 entry:
  :bar => 3

julia> foo[
!                        Channel                   TypeError                 delete!                   isabstract                numerator                 size
!=                       Char                      TypeVar                   deleteat!                 isabstracttype            object_id                 sizehint!
!==                      Cint                      UInt                      denominator               isalnum                   objectid                  sizeof
%                        Cintmax_t                 UInt128                   detach                    isalpha                   occursin      
...

[pfitzseb]

Mac FileWatching issue seems unrelated.

[StefanKarpinski]

Mac issue is likely #26725.