Fix string_slice with empty value

[apelisse]

Currently, if you don't set any value for a StringSliceVar, it will
end-up failing with an EOF error. It fails because the CSV parser for
the value can't read anything.

Special case when the string is empty to return an empty list.

cc @eparis @foxish

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[apelisse]

I re-pushed the commit with a test.

[eparis]

I would merge, but jenkins is showing me a blank page... will check back in a bit to see if jenkins is working...