Loosen the type restrictions of unit(::Type{Missing,T})

As suggested by @sostock, do not only restrict the type of T to Number, but include FixedPeriod as well, to accommodate Dates-soecific code.

Co-authored-by: Sebastian Stock <42280794+sostock@users.noreply.github.com>

Author: SeSodesa

src/utils.jl

@@ -143,7 +143,7 @@ true
 @inline unit(x::Number) = NoUnits
 @inline unit(x::Type{T}) where {T <: Number} = NoUnits
 @inline unit(x::Function) = NoUnits # Functions do not have units. Function values might.
-@inline unit(x::Type{Union{Missing, T}}) where { T <: Number } = unit(T) # Type restriction required here for T, or might result in infinite recursion.
+@inline unit(x::Type{Union{Missing, T}}) where {T <: Union{Number, FixedPeriod}} = unit(T) # Type restriction required here for T, or might result in infinite recursion.
 @inline unit(x::Type{Missing}) = missing
 @inline unit(x::Missing) = missing