Allow creating empty Table

[adkabo]

julia> Table()
ERROR: UndefVarError: n not defined
Stacktrace:
 [1] _ndims(#unused#::Type{Tuple{}})
   @ TypedTables ~/.julia/packages/TypedTables/zfbS2/src/TypedTables.jl:26

[andyferris]

This would be good - but zero-column tables aren’t supported yet. (We need to store a mutable copy of the indices somewhere for that).

[BeastyBlacksmith]

A better error message would help in the meantime, though.