issue_313: empty string is necessary in NcDataReader
See https://github.com/RWTH-EBC/AixLib/issues/313#issuecomment-278107763 @mlauster: Could you please merge these tiny changes.
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See https://github.com/RWTH-EBC/AixLib/issues/313#issuecomment-278107763 @mlauster: Could you please merge these tiny changes.