Fixed an extra comma added to a null cell when exporting CSV.

[tz850]

for #351

[jingwood]

Thanks! I have modified this pull request to process empty cells.
I think we should output the empty cells like a,b,,c rather than insert a space a,b, ,c.

And after some check, I found the original export logical is too simple and is not enough to process cell value which contains double quotes or commas, the code also has been improved.

There is a todo on how correctly read from CSV format when values contain double quotes or commas.

Refer to csv format on wikipedia