Fix allocatecolumn (#298)

* Fix allocatecolumn

* Update src/fallbacks.jl

* Update src/fallbacks.jl

src/fallbacks.jl

@@ -103,8 +103,13 @@ end
 
 Custom column types can override with an appropriate "scalar" element type that should dispatch to their column allocator.
 Alternatively, and more generally, custom scalars can overload `DataAPI.defaultarray` to signal the default array type.
+In this case the signaled array type must support a constructor accepting `undef` for initialization.
 """
-allocatecolumn(T, len) = DataAPI.defaultarray(T, 1)(Base.nonmissingtype(T) === T ? undef : missing, len)
+function allocatecolumn(T, len)
+    a = DataAPI.defaultarray(T, 1)(undef, len)
+    Missing <: T && fill!(a, missing)
+    return a
+end
 
 @inline function _allocatecolumns(::Schema{names, types}, len) where {names, types}
     if @generated