[Csv] Sniffer empty delimiter

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    >> For that case maybe we can define an optional default arg that is a
    >> single character.  Failing all other tests, the default is returned.

    Fred> The default shouldn't be type-checked (including string length),
    Fred> but should simply be returned if provided.  This allows the caller
    Fred> to determine the significance of getting back the passed-in value.

Hmmm...  To preserve current (incorrect?) behavior I think the default
almost has to be "".  To be useful though, it has to be a single-character
string given the current limitations of the module.

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