Allow DataAPI itself to own datavaluetype, nondatavaluetype, and unwrap

Author: quinnj

src/DataAPI.jl

@@ -72,11 +72,14 @@ For a type `T`, return the corresponding non-`DataValue` type, translating betwe
 For example, `nondatavaluetype(Int64)` returns `Int64`, while
 `nondatavaluetype(DataValue{Int64})` returns `Union{Int64, Missing}`.
 
-This generic function is owned by Tables.jl, which is the sole provider of the default
-definition.
+This generic function is owned by DataAPI.jl itself, which is the sole provider of the
+default definition.
 """
 function nondatavaluetype end
 
+nondatavaluetype(::Type{T}) where {T} = T
+nondatavaluetype(::Type{Union{}}) = Union{}
+
 """
     datavaluetype(T)
 
@@ -86,21 +89,26 @@ For a type `T`, return the corresponding `DataValue` type, translating between
 For example, `datavaluetype(Int64)` returns `Int64`, while
 `datavaluetype(Union{Int64, Missing})` returns `DataValue{Int64}`.
 
-This generic function is owned by Tables.jl, which is the sole provider of the default
-definition.
+This generic function is owned by DataAPI.jl itself, which is the sole provider of the
+default definition.
 """
 function datavaluetype end
 
+datavaluetype(::Type{T}) where {T} = T
+datavaluetype(::Type{Union{}}) = Union{}
+
 """
     unwrap(x)
 
 For a value `x`, potentially "unwrap" it from a `DataValue` or similar container.
 
-This generic function is owned by Tables.jl, which is the sole provider of the default
-definition.
+This generic function is owned by DataAPI.jl itself, which is the sole provider of the
+default definition.
 """
 function unwrap end
 
+unwrap(x) = x
+
 """
     describe(io::IO, x)