0-length index into a `StepRange` of `TimePeriod` errors

[tiemvanderdeure]

For example:

using Dates
(Dates.Hour(1):Dates.Hour(1):Dates.Hour(2))[1:0]

Errors with

ERROR: MethodError: no method matching copysign(::Hour, ::Hour)
The function `copysign` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  copysign(::Float64, ::Float64)
   @ Base floatfuncs.jl:5
  copysign(::Float64, ::Real)
   @ Base floatfuncs.jl:8
  copysign(::Float32, ::Float32)
   @ Base floatfuncs.jl:6
  ...

Stacktrace:
 [1] getindex(r::StepRange{Hour, Hour}, s::UnitRange{Int64})
   @ Base .\range.jl:1029
 [2] top-level scope

A possible fix would be to define Base.copysign on TimePeriod. The below works

import Dates.value
Base.copysign(x::T, y::T) where T<:TimePeriod = T(copysign(value(x), value(y)))

(Dates.Hour(1):Dates.Hour(1):Dates.Hour(2))[1:0]