Calling print on nothing is deprecated on Julia v0.7.0, errors on Julia v1.0.0

[jaakkor2]

using DataFrames, CSV
aa = randn(Float32, 10, 10)
fn_debug = "debug.csv"
CSV.write(fn_debug, DataFrame(aa))

gives on Julia v0.7.0

        dateformat: ┌ Warning: Calling `print` on `nothing` is deprecated; use `show`, `repr`, or custom output instead.

and on Julia v1.0.0

        dateformat: Error showing value of type CSV.Sink{Nothing,DataType}:
ERROR: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.

[nalimilan]

Good catch. Fixed by 6acdd19.