better handling of empty file in readdlm.

fixes #6898

With this patch:

````
julia> readcsv(IOBuffer(""))
ERROR: Empty input
 in DLMStore at datafmt.jl:153
 in dlm_fill at datafmt.jl:294
 in readdlm_string at datafmt.jl:270
 in readdlm_auto at datafmt.jl:58
 in readdlm at datafmt.jl:49
 in readdlm at datafmt.jl:47
 in readcsv at datafmt.jl:470

julia> readcsv(IOBuffer("\n"))
ERROR: Empty input
 in DLMStore at datafmt.jl:153
 in dlm_fill at datafmt.jl:294
 in readdlm_string at datafmt.jl:270
 in readdlm_auto at datafmt.jl:58
 in readdlm at datafmt.jl:49
 in readdlm at datafmt.jl:47
 in readcsv at datafmt.jl:470

julia> readcsv(IOBuffer("\n\n\n"))
ERROR: Empty input
 in DLMStore at datafmt.jl:153
 in dlm_fill at datafmt.jl:294
 in readdlm_string at datafmt.jl:270
 in readdlm_auto at datafmt.jl:58
 in readdlm at datafmt.jl:49
 in readdlm at datafmt.jl:47
 in readcsv at datafmt.jl:470
````