... without generating  0;;;@  from  0;@  that has different semantics.

Introduce css::util::NumberFormat::EMPTY to properly flag empty
subformats and distinguish from UNDEFINED, everything else would be an
ugly hack.

SvNumberformat::GetMappedFormatstring() now correctly supports the
trailing text subformat, so exports using it should get that
automatically.

Change-Id: If9a1bcc5ec5dfcf46688035e2b1428ab4747a68d