Avoid copying input Array{Union{T, Null}} in DataArray{T} constructor

[nalimilan]

We currently make a copy of an Array{Union{T, Null}} input to an Array{T} because the data field of DataArray should be Array{T}. We should use low-level Julia API once available to access the underlying value part of the input as an Array{T} (for isbits types) in an unsafe way, since we know we will never access null entries. See this discussion.